Why is 0 ft a valid target altitude for an autopilot?









up vote
27
down vote

favorite












Inspired by this story;




The pilot of a Flybe plane that dropped 500 ft (152m) in 18 seconds had
selected the wrong autopilot setting shortly after take-off, an
inquiry has found...



...The plane climbed to 1,500 ft, but then pitched and "descended
rapidly" because the autopilot was set with a target altitude of 0 ft.




Why is 0 ft a valid target value for the autopilot? Is there any practical use for being able to set it so low and are there safety features that would prevent the plane just hitting the deck?










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  • 31




    To cruise at low altitude in Death Valley?
    – quiet flyer
    2 days ago






  • 3




    @quietflyer OK, good point that the ground level might be below 0ft sea level, I hadn’t thought of that. But in my head I imagined there would be some kind of buffer below where the autopilot couldn’t be trusted.
    – Darren
    2 days ago






  • 2




    Yes. me too, that's why I didn't form it into a real answer. Good question.
    – quiet flyer
    2 days ago






  • 4




    Which plane was that? Dash8? Bae146 or ATR? After it will depend of the autopilot coupled with the FD. For Airbus and Boeing the minimum is 100ft. But the autopilot is capable to land the aircraft so, it is able to go below 0ft ASL if you land in Amsterdam for example.
    – Chris Lau
    2 days ago






  • 3




    @ChrisLau - during auto land in the Boeing, I believe that at GS capture the Go Around mode is armed and, depending on company procedures, the missed approach altitude is (pre)set in the altitude selector. (because the AP is locked on the approach and won't go to the altitude in the MCP altitude selected unless GA (toga) is pushed).
    – 757toga
    2 days ago















up vote
27
down vote

favorite












Inspired by this story;




The pilot of a Flybe plane that dropped 500 ft (152m) in 18 seconds had
selected the wrong autopilot setting shortly after take-off, an
inquiry has found...



...The plane climbed to 1,500 ft, but then pitched and "descended
rapidly" because the autopilot was set with a target altitude of 0 ft.




Why is 0 ft a valid target value for the autopilot? Is there any practical use for being able to set it so low and are there safety features that would prevent the plane just hitting the deck?










share|improve this question









New contributor




Darren is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.















  • 31




    To cruise at low altitude in Death Valley?
    – quiet flyer
    2 days ago






  • 3




    @quietflyer OK, good point that the ground level might be below 0ft sea level, I hadn’t thought of that. But in my head I imagined there would be some kind of buffer below where the autopilot couldn’t be trusted.
    – Darren
    2 days ago






  • 2




    Yes. me too, that's why I didn't form it into a real answer. Good question.
    – quiet flyer
    2 days ago






  • 4




    Which plane was that? Dash8? Bae146 or ATR? After it will depend of the autopilot coupled with the FD. For Airbus and Boeing the minimum is 100ft. But the autopilot is capable to land the aircraft so, it is able to go below 0ft ASL if you land in Amsterdam for example.
    – Chris Lau
    2 days ago






  • 3




    @ChrisLau - during auto land in the Boeing, I believe that at GS capture the Go Around mode is armed and, depending on company procedures, the missed approach altitude is (pre)set in the altitude selector. (because the AP is locked on the approach and won't go to the altitude in the MCP altitude selected unless GA (toga) is pushed).
    – 757toga
    2 days ago













up vote
27
down vote

favorite









up vote
27
down vote

favorite











Inspired by this story;




The pilot of a Flybe plane that dropped 500 ft (152m) in 18 seconds had
selected the wrong autopilot setting shortly after take-off, an
inquiry has found...



...The plane climbed to 1,500 ft, but then pitched and "descended
rapidly" because the autopilot was set with a target altitude of 0 ft.




Why is 0 ft a valid target value for the autopilot? Is there any practical use for being able to set it so low and are there safety features that would prevent the plane just hitting the deck?










share|improve this question









New contributor




Darren is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.











Inspired by this story;




The pilot of a Flybe plane that dropped 500 ft (152m) in 18 seconds had
selected the wrong autopilot setting shortly after take-off, an
inquiry has found...



...The plane climbed to 1,500 ft, but then pitched and "descended
rapidly" because the autopilot was set with a target altitude of 0 ft.




Why is 0 ft a valid target value for the autopilot? Is there any practical use for being able to set it so low and are there safety features that would prevent the plane just hitting the deck?







autopilot altitude






share|improve this question









New contributor




Darren is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.











share|improve this question









New contributor




Darren is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.









share|improve this question




share|improve this question








edited 23 hours ago









user71659

2,184617




2,184617






New contributor




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asked 2 days ago









Darren

23634




23634




New contributor




Darren is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.





New contributor





Darren is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.






Darren is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.







  • 31




    To cruise at low altitude in Death Valley?
    – quiet flyer
    2 days ago






  • 3




    @quietflyer OK, good point that the ground level might be below 0ft sea level, I hadn’t thought of that. But in my head I imagined there would be some kind of buffer below where the autopilot couldn’t be trusted.
    – Darren
    2 days ago






  • 2




    Yes. me too, that's why I didn't form it into a real answer. Good question.
    – quiet flyer
    2 days ago






  • 4




    Which plane was that? Dash8? Bae146 or ATR? After it will depend of the autopilot coupled with the FD. For Airbus and Boeing the minimum is 100ft. But the autopilot is capable to land the aircraft so, it is able to go below 0ft ASL if you land in Amsterdam for example.
    – Chris Lau
    2 days ago






  • 3




    @ChrisLau - during auto land in the Boeing, I believe that at GS capture the Go Around mode is armed and, depending on company procedures, the missed approach altitude is (pre)set in the altitude selector. (because the AP is locked on the approach and won't go to the altitude in the MCP altitude selected unless GA (toga) is pushed).
    – 757toga
    2 days ago













  • 31




    To cruise at low altitude in Death Valley?
    – quiet flyer
    2 days ago






  • 3




    @quietflyer OK, good point that the ground level might be below 0ft sea level, I hadn’t thought of that. But in my head I imagined there would be some kind of buffer below where the autopilot couldn’t be trusted.
    – Darren
    2 days ago






  • 2




    Yes. me too, that's why I didn't form it into a real answer. Good question.
    – quiet flyer
    2 days ago






  • 4




    Which plane was that? Dash8? Bae146 or ATR? After it will depend of the autopilot coupled with the FD. For Airbus and Boeing the minimum is 100ft. But the autopilot is capable to land the aircraft so, it is able to go below 0ft ASL if you land in Amsterdam for example.
    – Chris Lau
    2 days ago






  • 3




    @ChrisLau - during auto land in the Boeing, I believe that at GS capture the Go Around mode is armed and, depending on company procedures, the missed approach altitude is (pre)set in the altitude selector. (because the AP is locked on the approach and won't go to the altitude in the MCP altitude selected unless GA (toga) is pushed).
    – 757toga
    2 days ago








31




31




To cruise at low altitude in Death Valley?
– quiet flyer
2 days ago




To cruise at low altitude in Death Valley?
– quiet flyer
2 days ago




3




3




@quietflyer OK, good point that the ground level might be below 0ft sea level, I hadn’t thought of that. But in my head I imagined there would be some kind of buffer below where the autopilot couldn’t be trusted.
– Darren
2 days ago




@quietflyer OK, good point that the ground level might be below 0ft sea level, I hadn’t thought of that. But in my head I imagined there would be some kind of buffer below where the autopilot couldn’t be trusted.
– Darren
2 days ago




2




2




Yes. me too, that's why I didn't form it into a real answer. Good question.
– quiet flyer
2 days ago




Yes. me too, that's why I didn't form it into a real answer. Good question.
– quiet flyer
2 days ago




4




4




Which plane was that? Dash8? Bae146 or ATR? After it will depend of the autopilot coupled with the FD. For Airbus and Boeing the minimum is 100ft. But the autopilot is capable to land the aircraft so, it is able to go below 0ft ASL if you land in Amsterdam for example.
– Chris Lau
2 days ago




Which plane was that? Dash8? Bae146 or ATR? After it will depend of the autopilot coupled with the FD. For Airbus and Boeing the minimum is 100ft. But the autopilot is capable to land the aircraft so, it is able to go below 0ft ASL if you land in Amsterdam for example.
– Chris Lau
2 days ago




3




3




@ChrisLau - during auto land in the Boeing, I believe that at GS capture the Go Around mode is armed and, depending on company procedures, the missed approach altitude is (pre)set in the altitude selector. (because the AP is locked on the approach and won't go to the altitude in the MCP altitude selected unless GA (toga) is pushed).
– 757toga
2 days ago





@ChrisLau - during auto land in the Boeing, I believe that at GS capture the Go Around mode is armed and, depending on company procedures, the missed approach altitude is (pre)set in the altitude selector. (because the AP is locked on the approach and won't go to the altitude in the MCP altitude selected unless GA (toga) is pushed).
– 757toga
2 days ago











3 Answers
3






active

oldest

votes

















up vote
35
down vote













Quite simply, it's because sometimes you fly below sea level. There's a couple stories out there of aircraft navigation systems acting a bit odd due to their flight below sea level. For example, there is this one, which involves a C-130 landing on an airfield that is 1,210 feet below sea level.



There are numerous areas in the world that are below sea level. For example, flying at sealevel above Laguna Salada, Mexico means you are still 30 feet above the ground! It's added as an additional option to cover a small set of scenarios. A pilot is certainly not expected to select the incorrect altitude setting, as this is something that you should always doublecheck. Beyond that, he should not have enabled his autopilot so soon after takeoff (unless 1,500 was his cruising altitude).






share|improve this answer


















  • 2




    It's more than acceptable to engage your autopilot at 1,500 feet in most planes. Most of the ones that I fly specify 1,000 feet as the minimum, and in busy airspace or when you want to devote more attention to avoiding weather, it can even be a good idea.
    – Lnafziger
    2 days ago






  • 2




    You can use the autopilot as soon as 400 ft AGL on the 737NG, and on the A320, its 100 ft AGL! (On the A320 though, you have to be already using the flight director/SRS, so you know it's going do something sensible)
    – user71659
    2 days ago






  • 2




    Regarding the people who says "it's acceptable to engage autopilot at 1,500 feet," - what is acceptable and what is recommended, are two different things. I can tell you first hand commercial customers are typically told to climb manually, and engage autopilot at altitude. Whether they heed that advice is to their discretion. My instructor certainly would never have allowed me to.
    – Matt
    yesterday






  • 1




    @Matt No way on an airliner, since a jet climb is through RVSM airspace, it would be illegal for you to hand-fly to altitude. Airline operations would expect you to be on AP by 1000 ft AGL, and there are cases like RNP AR which require you to be on AP as soon as 400 ft AGL.
    – user71659
    yesterday










  • Yeah, the Jordan River Valley is an interesting place. It's interesting to be on a mountain looking 1,000 feet down at the surrounding area while yourself still being below sea level.
    – reirab
    23 hours ago

















up vote
12
down vote













The answer is no you would never depart with the altitude preselect at 0. You would normally set the initial level off altitude provided in your departure clearance, or an initial level off altitude indicated in the SID, or you might set a level off altitude that you have decided to use as a level off for the single engine profile in the event of engine failure, based on the terrain around the airport. All would be at least a 1000 ft above the airport at minimum.



You dial up that altitude in the AP's altitude preselect window, usually during the configuration setup during or after the crew departure briefing. When you are airborne, you will normally select the autopilot's speed hold mode (on the -8 AP it's called IAS mode) which makes the flight director command a pitch attitude to hold bug speed, then when you pass 400 feet or above, you engage A/P in IAS mode and it will pitch the plane to hold the bug speed you select during the departure. In IAS mode, whether you climb or descend depends on the power setting, so when you are climbing and descending in IAS mode you regulate the climb/descent with power while the AP pitches to hold the speed. You can also use VS mode, or vertical speed mode, and the AP will now pitch to a preset vertical climb or descent rate.



With those two vertical modes, the incident description doesn't make a lot of sense as they would have had to have selected VS mode and dialed in a descent rate, or been in IAS mode and cut the power. However...



The -8 400 has coupled VNAV, which adds an extra wrinkle. With VNAV engaged, the autopilot will pitch to climb or descend to an altitude preselect independently of IAS or VS mode, instead of just hold an airspeed or vertical speed, until a preselect altitude is captured. Coupled VNAV can also follow step climbs in a SID or step descents in a STAR if that is programmed into the FMS.



So my guess here is that they forgot to set the altitude preselect for their planned departure level off altitude, then engaged the autopilot in VNAV mode during the climbout, which caused the plane to dive to the existing altitude preselect of 0 feet, upon which hilarity ensued.






share|improve this answer


















  • 11




    For certain values of "hilarity."
    – Zach Lipton
    2 days ago










  • Nice answer. Thanks.
    – Darren
    2 days ago










  • "Hilarity ensues" is a line from the old Frasier sitcom tv show when he describes his plan to play a practical joke on his boss. One of my favourite bits.
    – John K
    yesterday










  • "The answer is no you would never depart with the altitude preselect at 0." It's entirely possible for 0 MSL to be more than 1,000 feet above field level. This is possible by the Dead Sea, for example.
    – reirab
    23 hours ago











  • @JohnK: The phrase may have been popularized by Frasier, but it is far older than that. Here's an example from 1872: books.google.com/…
    – Eric Lippert
    13 hours ago

















up vote
7
down vote













Besides flying below sea level, a situation for low and negative altimeter readings is use of QFE altimetry. (see also What is wrong about this interpretation of QFE and QNH?)



QFE altimetry which was used by some airlines (gone by now), and is still in use in Russia (undergoing phase-out), is where you set the altimeter's reference pressure so that it reads zero at some ground reference point, typically the runway threshold.



So if the runway is above you, on terrain, you will fly at a negative height and your altimeters will show as such.






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    3 Answers
    3






    active

    oldest

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    3 Answers
    3






    active

    oldest

    votes









    active

    oldest

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    active

    oldest

    votes








    up vote
    35
    down vote













    Quite simply, it's because sometimes you fly below sea level. There's a couple stories out there of aircraft navigation systems acting a bit odd due to their flight below sea level. For example, there is this one, which involves a C-130 landing on an airfield that is 1,210 feet below sea level.



    There are numerous areas in the world that are below sea level. For example, flying at sealevel above Laguna Salada, Mexico means you are still 30 feet above the ground! It's added as an additional option to cover a small set of scenarios. A pilot is certainly not expected to select the incorrect altitude setting, as this is something that you should always doublecheck. Beyond that, he should not have enabled his autopilot so soon after takeoff (unless 1,500 was his cruising altitude).






    share|improve this answer


















    • 2




      It's more than acceptable to engage your autopilot at 1,500 feet in most planes. Most of the ones that I fly specify 1,000 feet as the minimum, and in busy airspace or when you want to devote more attention to avoiding weather, it can even be a good idea.
      – Lnafziger
      2 days ago






    • 2




      You can use the autopilot as soon as 400 ft AGL on the 737NG, and on the A320, its 100 ft AGL! (On the A320 though, you have to be already using the flight director/SRS, so you know it's going do something sensible)
      – user71659
      2 days ago






    • 2




      Regarding the people who says "it's acceptable to engage autopilot at 1,500 feet," - what is acceptable and what is recommended, are two different things. I can tell you first hand commercial customers are typically told to climb manually, and engage autopilot at altitude. Whether they heed that advice is to their discretion. My instructor certainly would never have allowed me to.
      – Matt
      yesterday






    • 1




      @Matt No way on an airliner, since a jet climb is through RVSM airspace, it would be illegal for you to hand-fly to altitude. Airline operations would expect you to be on AP by 1000 ft AGL, and there are cases like RNP AR which require you to be on AP as soon as 400 ft AGL.
      – user71659
      yesterday










    • Yeah, the Jordan River Valley is an interesting place. It's interesting to be on a mountain looking 1,000 feet down at the surrounding area while yourself still being below sea level.
      – reirab
      23 hours ago














    up vote
    35
    down vote













    Quite simply, it's because sometimes you fly below sea level. There's a couple stories out there of aircraft navigation systems acting a bit odd due to their flight below sea level. For example, there is this one, which involves a C-130 landing on an airfield that is 1,210 feet below sea level.



    There are numerous areas in the world that are below sea level. For example, flying at sealevel above Laguna Salada, Mexico means you are still 30 feet above the ground! It's added as an additional option to cover a small set of scenarios. A pilot is certainly not expected to select the incorrect altitude setting, as this is something that you should always doublecheck. Beyond that, he should not have enabled his autopilot so soon after takeoff (unless 1,500 was his cruising altitude).






    share|improve this answer


















    • 2




      It's more than acceptable to engage your autopilot at 1,500 feet in most planes. Most of the ones that I fly specify 1,000 feet as the minimum, and in busy airspace or when you want to devote more attention to avoiding weather, it can even be a good idea.
      – Lnafziger
      2 days ago






    • 2




      You can use the autopilot as soon as 400 ft AGL on the 737NG, and on the A320, its 100 ft AGL! (On the A320 though, you have to be already using the flight director/SRS, so you know it's going do something sensible)
      – user71659
      2 days ago






    • 2




      Regarding the people who says "it's acceptable to engage autopilot at 1,500 feet," - what is acceptable and what is recommended, are two different things. I can tell you first hand commercial customers are typically told to climb manually, and engage autopilot at altitude. Whether they heed that advice is to their discretion. My instructor certainly would never have allowed me to.
      – Matt
      yesterday






    • 1




      @Matt No way on an airliner, since a jet climb is through RVSM airspace, it would be illegal for you to hand-fly to altitude. Airline operations would expect you to be on AP by 1000 ft AGL, and there are cases like RNP AR which require you to be on AP as soon as 400 ft AGL.
      – user71659
      yesterday










    • Yeah, the Jordan River Valley is an interesting place. It's interesting to be on a mountain looking 1,000 feet down at the surrounding area while yourself still being below sea level.
      – reirab
      23 hours ago












    up vote
    35
    down vote










    up vote
    35
    down vote









    Quite simply, it's because sometimes you fly below sea level. There's a couple stories out there of aircraft navigation systems acting a bit odd due to their flight below sea level. For example, there is this one, which involves a C-130 landing on an airfield that is 1,210 feet below sea level.



    There are numerous areas in the world that are below sea level. For example, flying at sealevel above Laguna Salada, Mexico means you are still 30 feet above the ground! It's added as an additional option to cover a small set of scenarios. A pilot is certainly not expected to select the incorrect altitude setting, as this is something that you should always doublecheck. Beyond that, he should not have enabled his autopilot so soon after takeoff (unless 1,500 was his cruising altitude).






    share|improve this answer














    Quite simply, it's because sometimes you fly below sea level. There's a couple stories out there of aircraft navigation systems acting a bit odd due to their flight below sea level. For example, there is this one, which involves a C-130 landing on an airfield that is 1,210 feet below sea level.



    There are numerous areas in the world that are below sea level. For example, flying at sealevel above Laguna Salada, Mexico means you are still 30 feet above the ground! It's added as an additional option to cover a small set of scenarios. A pilot is certainly not expected to select the incorrect altitude setting, as this is something that you should always doublecheck. Beyond that, he should not have enabled his autopilot so soon after takeoff (unless 1,500 was his cruising altitude).







    share|improve this answer














    share|improve this answer



    share|improve this answer








    edited 2 days ago









    Mark Omo

    1054




    1054










    answered 2 days ago









    Matt

    1,591520




    1,591520







    • 2




      It's more than acceptable to engage your autopilot at 1,500 feet in most planes. Most of the ones that I fly specify 1,000 feet as the minimum, and in busy airspace or when you want to devote more attention to avoiding weather, it can even be a good idea.
      – Lnafziger
      2 days ago






    • 2




      You can use the autopilot as soon as 400 ft AGL on the 737NG, and on the A320, its 100 ft AGL! (On the A320 though, you have to be already using the flight director/SRS, so you know it's going do something sensible)
      – user71659
      2 days ago






    • 2




      Regarding the people who says "it's acceptable to engage autopilot at 1,500 feet," - what is acceptable and what is recommended, are two different things. I can tell you first hand commercial customers are typically told to climb manually, and engage autopilot at altitude. Whether they heed that advice is to their discretion. My instructor certainly would never have allowed me to.
      – Matt
      yesterday






    • 1




      @Matt No way on an airliner, since a jet climb is through RVSM airspace, it would be illegal for you to hand-fly to altitude. Airline operations would expect you to be on AP by 1000 ft AGL, and there are cases like RNP AR which require you to be on AP as soon as 400 ft AGL.
      – user71659
      yesterday










    • Yeah, the Jordan River Valley is an interesting place. It's interesting to be on a mountain looking 1,000 feet down at the surrounding area while yourself still being below sea level.
      – reirab
      23 hours ago












    • 2




      It's more than acceptable to engage your autopilot at 1,500 feet in most planes. Most of the ones that I fly specify 1,000 feet as the minimum, and in busy airspace or when you want to devote more attention to avoiding weather, it can even be a good idea.
      – Lnafziger
      2 days ago






    • 2




      You can use the autopilot as soon as 400 ft AGL on the 737NG, and on the A320, its 100 ft AGL! (On the A320 though, you have to be already using the flight director/SRS, so you know it's going do something sensible)
      – user71659
      2 days ago






    • 2




      Regarding the people who says "it's acceptable to engage autopilot at 1,500 feet," - what is acceptable and what is recommended, are two different things. I can tell you first hand commercial customers are typically told to climb manually, and engage autopilot at altitude. Whether they heed that advice is to their discretion. My instructor certainly would never have allowed me to.
      – Matt
      yesterday






    • 1




      @Matt No way on an airliner, since a jet climb is through RVSM airspace, it would be illegal for you to hand-fly to altitude. Airline operations would expect you to be on AP by 1000 ft AGL, and there are cases like RNP AR which require you to be on AP as soon as 400 ft AGL.
      – user71659
      yesterday










    • Yeah, the Jordan River Valley is an interesting place. It's interesting to be on a mountain looking 1,000 feet down at the surrounding area while yourself still being below sea level.
      – reirab
      23 hours ago







    2




    2




    It's more than acceptable to engage your autopilot at 1,500 feet in most planes. Most of the ones that I fly specify 1,000 feet as the minimum, and in busy airspace or when you want to devote more attention to avoiding weather, it can even be a good idea.
    – Lnafziger
    2 days ago




    It's more than acceptable to engage your autopilot at 1,500 feet in most planes. Most of the ones that I fly specify 1,000 feet as the minimum, and in busy airspace or when you want to devote more attention to avoiding weather, it can even be a good idea.
    – Lnafziger
    2 days ago




    2




    2




    You can use the autopilot as soon as 400 ft AGL on the 737NG, and on the A320, its 100 ft AGL! (On the A320 though, you have to be already using the flight director/SRS, so you know it's going do something sensible)
    – user71659
    2 days ago




    You can use the autopilot as soon as 400 ft AGL on the 737NG, and on the A320, its 100 ft AGL! (On the A320 though, you have to be already using the flight director/SRS, so you know it's going do something sensible)
    – user71659
    2 days ago




    2




    2




    Regarding the people who says "it's acceptable to engage autopilot at 1,500 feet," - what is acceptable and what is recommended, are two different things. I can tell you first hand commercial customers are typically told to climb manually, and engage autopilot at altitude. Whether they heed that advice is to their discretion. My instructor certainly would never have allowed me to.
    – Matt
    yesterday




    Regarding the people who says "it's acceptable to engage autopilot at 1,500 feet," - what is acceptable and what is recommended, are two different things. I can tell you first hand commercial customers are typically told to climb manually, and engage autopilot at altitude. Whether they heed that advice is to their discretion. My instructor certainly would never have allowed me to.
    – Matt
    yesterday




    1




    1




    @Matt No way on an airliner, since a jet climb is through RVSM airspace, it would be illegal for you to hand-fly to altitude. Airline operations would expect you to be on AP by 1000 ft AGL, and there are cases like RNP AR which require you to be on AP as soon as 400 ft AGL.
    – user71659
    yesterday




    @Matt No way on an airliner, since a jet climb is through RVSM airspace, it would be illegal for you to hand-fly to altitude. Airline operations would expect you to be on AP by 1000 ft AGL, and there are cases like RNP AR which require you to be on AP as soon as 400 ft AGL.
    – user71659
    yesterday












    Yeah, the Jordan River Valley is an interesting place. It's interesting to be on a mountain looking 1,000 feet down at the surrounding area while yourself still being below sea level.
    – reirab
    23 hours ago




    Yeah, the Jordan River Valley is an interesting place. It's interesting to be on a mountain looking 1,000 feet down at the surrounding area while yourself still being below sea level.
    – reirab
    23 hours ago










    up vote
    12
    down vote













    The answer is no you would never depart with the altitude preselect at 0. You would normally set the initial level off altitude provided in your departure clearance, or an initial level off altitude indicated in the SID, or you might set a level off altitude that you have decided to use as a level off for the single engine profile in the event of engine failure, based on the terrain around the airport. All would be at least a 1000 ft above the airport at minimum.



    You dial up that altitude in the AP's altitude preselect window, usually during the configuration setup during or after the crew departure briefing. When you are airborne, you will normally select the autopilot's speed hold mode (on the -8 AP it's called IAS mode) which makes the flight director command a pitch attitude to hold bug speed, then when you pass 400 feet or above, you engage A/P in IAS mode and it will pitch the plane to hold the bug speed you select during the departure. In IAS mode, whether you climb or descend depends on the power setting, so when you are climbing and descending in IAS mode you regulate the climb/descent with power while the AP pitches to hold the speed. You can also use VS mode, or vertical speed mode, and the AP will now pitch to a preset vertical climb or descent rate.



    With those two vertical modes, the incident description doesn't make a lot of sense as they would have had to have selected VS mode and dialed in a descent rate, or been in IAS mode and cut the power. However...



    The -8 400 has coupled VNAV, which adds an extra wrinkle. With VNAV engaged, the autopilot will pitch to climb or descend to an altitude preselect independently of IAS or VS mode, instead of just hold an airspeed or vertical speed, until a preselect altitude is captured. Coupled VNAV can also follow step climbs in a SID or step descents in a STAR if that is programmed into the FMS.



    So my guess here is that they forgot to set the altitude preselect for their planned departure level off altitude, then engaged the autopilot in VNAV mode during the climbout, which caused the plane to dive to the existing altitude preselect of 0 feet, upon which hilarity ensued.






    share|improve this answer


















    • 11




      For certain values of "hilarity."
      – Zach Lipton
      2 days ago










    • Nice answer. Thanks.
      – Darren
      2 days ago










    • "Hilarity ensues" is a line from the old Frasier sitcom tv show when he describes his plan to play a practical joke on his boss. One of my favourite bits.
      – John K
      yesterday










    • "The answer is no you would never depart with the altitude preselect at 0." It's entirely possible for 0 MSL to be more than 1,000 feet above field level. This is possible by the Dead Sea, for example.
      – reirab
      23 hours ago











    • @JohnK: The phrase may have been popularized by Frasier, but it is far older than that. Here's an example from 1872: books.google.com/…
      – Eric Lippert
      13 hours ago














    up vote
    12
    down vote













    The answer is no you would never depart with the altitude preselect at 0. You would normally set the initial level off altitude provided in your departure clearance, or an initial level off altitude indicated in the SID, or you might set a level off altitude that you have decided to use as a level off for the single engine profile in the event of engine failure, based on the terrain around the airport. All would be at least a 1000 ft above the airport at minimum.



    You dial up that altitude in the AP's altitude preselect window, usually during the configuration setup during or after the crew departure briefing. When you are airborne, you will normally select the autopilot's speed hold mode (on the -8 AP it's called IAS mode) which makes the flight director command a pitch attitude to hold bug speed, then when you pass 400 feet or above, you engage A/P in IAS mode and it will pitch the plane to hold the bug speed you select during the departure. In IAS mode, whether you climb or descend depends on the power setting, so when you are climbing and descending in IAS mode you regulate the climb/descent with power while the AP pitches to hold the speed. You can also use VS mode, or vertical speed mode, and the AP will now pitch to a preset vertical climb or descent rate.



    With those two vertical modes, the incident description doesn't make a lot of sense as they would have had to have selected VS mode and dialed in a descent rate, or been in IAS mode and cut the power. However...



    The -8 400 has coupled VNAV, which adds an extra wrinkle. With VNAV engaged, the autopilot will pitch to climb or descend to an altitude preselect independently of IAS or VS mode, instead of just hold an airspeed or vertical speed, until a preselect altitude is captured. Coupled VNAV can also follow step climbs in a SID or step descents in a STAR if that is programmed into the FMS.



    So my guess here is that they forgot to set the altitude preselect for their planned departure level off altitude, then engaged the autopilot in VNAV mode during the climbout, which caused the plane to dive to the existing altitude preselect of 0 feet, upon which hilarity ensued.






    share|improve this answer


















    • 11




      For certain values of "hilarity."
      – Zach Lipton
      2 days ago










    • Nice answer. Thanks.
      – Darren
      2 days ago










    • "Hilarity ensues" is a line from the old Frasier sitcom tv show when he describes his plan to play a practical joke on his boss. One of my favourite bits.
      – John K
      yesterday










    • "The answer is no you would never depart with the altitude preselect at 0." It's entirely possible for 0 MSL to be more than 1,000 feet above field level. This is possible by the Dead Sea, for example.
      – reirab
      23 hours ago











    • @JohnK: The phrase may have been popularized by Frasier, but it is far older than that. Here's an example from 1872: books.google.com/…
      – Eric Lippert
      13 hours ago












    up vote
    12
    down vote










    up vote
    12
    down vote









    The answer is no you would never depart with the altitude preselect at 0. You would normally set the initial level off altitude provided in your departure clearance, or an initial level off altitude indicated in the SID, or you might set a level off altitude that you have decided to use as a level off for the single engine profile in the event of engine failure, based on the terrain around the airport. All would be at least a 1000 ft above the airport at minimum.



    You dial up that altitude in the AP's altitude preselect window, usually during the configuration setup during or after the crew departure briefing. When you are airborne, you will normally select the autopilot's speed hold mode (on the -8 AP it's called IAS mode) which makes the flight director command a pitch attitude to hold bug speed, then when you pass 400 feet or above, you engage A/P in IAS mode and it will pitch the plane to hold the bug speed you select during the departure. In IAS mode, whether you climb or descend depends on the power setting, so when you are climbing and descending in IAS mode you regulate the climb/descent with power while the AP pitches to hold the speed. You can also use VS mode, or vertical speed mode, and the AP will now pitch to a preset vertical climb or descent rate.



    With those two vertical modes, the incident description doesn't make a lot of sense as they would have had to have selected VS mode and dialed in a descent rate, or been in IAS mode and cut the power. However...



    The -8 400 has coupled VNAV, which adds an extra wrinkle. With VNAV engaged, the autopilot will pitch to climb or descend to an altitude preselect independently of IAS or VS mode, instead of just hold an airspeed or vertical speed, until a preselect altitude is captured. Coupled VNAV can also follow step climbs in a SID or step descents in a STAR if that is programmed into the FMS.



    So my guess here is that they forgot to set the altitude preselect for their planned departure level off altitude, then engaged the autopilot in VNAV mode during the climbout, which caused the plane to dive to the existing altitude preselect of 0 feet, upon which hilarity ensued.






    share|improve this answer














    The answer is no you would never depart with the altitude preselect at 0. You would normally set the initial level off altitude provided in your departure clearance, or an initial level off altitude indicated in the SID, or you might set a level off altitude that you have decided to use as a level off for the single engine profile in the event of engine failure, based on the terrain around the airport. All would be at least a 1000 ft above the airport at minimum.



    You dial up that altitude in the AP's altitude preselect window, usually during the configuration setup during or after the crew departure briefing. When you are airborne, you will normally select the autopilot's speed hold mode (on the -8 AP it's called IAS mode) which makes the flight director command a pitch attitude to hold bug speed, then when you pass 400 feet or above, you engage A/P in IAS mode and it will pitch the plane to hold the bug speed you select during the departure. In IAS mode, whether you climb or descend depends on the power setting, so when you are climbing and descending in IAS mode you regulate the climb/descent with power while the AP pitches to hold the speed. You can also use VS mode, or vertical speed mode, and the AP will now pitch to a preset vertical climb or descent rate.



    With those two vertical modes, the incident description doesn't make a lot of sense as they would have had to have selected VS mode and dialed in a descent rate, or been in IAS mode and cut the power. However...



    The -8 400 has coupled VNAV, which adds an extra wrinkle. With VNAV engaged, the autopilot will pitch to climb or descend to an altitude preselect independently of IAS or VS mode, instead of just hold an airspeed or vertical speed, until a preselect altitude is captured. Coupled VNAV can also follow step climbs in a SID or step descents in a STAR if that is programmed into the FMS.



    So my guess here is that they forgot to set the altitude preselect for their planned departure level off altitude, then engaged the autopilot in VNAV mode during the climbout, which caused the plane to dive to the existing altitude preselect of 0 feet, upon which hilarity ensued.







    share|improve this answer














    share|improve this answer



    share|improve this answer








    edited 2 days ago

























    answered 2 days ago









    John K

    10.9k1134




    10.9k1134







    • 11




      For certain values of "hilarity."
      – Zach Lipton
      2 days ago










    • Nice answer. Thanks.
      – Darren
      2 days ago










    • "Hilarity ensues" is a line from the old Frasier sitcom tv show when he describes his plan to play a practical joke on his boss. One of my favourite bits.
      – John K
      yesterday










    • "The answer is no you would never depart with the altitude preselect at 0." It's entirely possible for 0 MSL to be more than 1,000 feet above field level. This is possible by the Dead Sea, for example.
      – reirab
      23 hours ago











    • @JohnK: The phrase may have been popularized by Frasier, but it is far older than that. Here's an example from 1872: books.google.com/…
      – Eric Lippert
      13 hours ago












    • 11




      For certain values of "hilarity."
      – Zach Lipton
      2 days ago










    • Nice answer. Thanks.
      – Darren
      2 days ago










    • "Hilarity ensues" is a line from the old Frasier sitcom tv show when he describes his plan to play a practical joke on his boss. One of my favourite bits.
      – John K
      yesterday










    • "The answer is no you would never depart with the altitude preselect at 0." It's entirely possible for 0 MSL to be more than 1,000 feet above field level. This is possible by the Dead Sea, for example.
      – reirab
      23 hours ago











    • @JohnK: The phrase may have been popularized by Frasier, but it is far older than that. Here's an example from 1872: books.google.com/…
      – Eric Lippert
      13 hours ago







    11




    11




    For certain values of "hilarity."
    – Zach Lipton
    2 days ago




    For certain values of "hilarity."
    – Zach Lipton
    2 days ago












    Nice answer. Thanks.
    – Darren
    2 days ago




    Nice answer. Thanks.
    – Darren
    2 days ago












    "Hilarity ensues" is a line from the old Frasier sitcom tv show when he describes his plan to play a practical joke on his boss. One of my favourite bits.
    – John K
    yesterday




    "Hilarity ensues" is a line from the old Frasier sitcom tv show when he describes his plan to play a practical joke on his boss. One of my favourite bits.
    – John K
    yesterday












    "The answer is no you would never depart with the altitude preselect at 0." It's entirely possible for 0 MSL to be more than 1,000 feet above field level. This is possible by the Dead Sea, for example.
    – reirab
    23 hours ago





    "The answer is no you would never depart with the altitude preselect at 0." It's entirely possible for 0 MSL to be more than 1,000 feet above field level. This is possible by the Dead Sea, for example.
    – reirab
    23 hours ago













    @JohnK: The phrase may have been popularized by Frasier, but it is far older than that. Here's an example from 1872: books.google.com/…
    – Eric Lippert
    13 hours ago




    @JohnK: The phrase may have been popularized by Frasier, but it is far older than that. Here's an example from 1872: books.google.com/…
    – Eric Lippert
    13 hours ago










    up vote
    7
    down vote













    Besides flying below sea level, a situation for low and negative altimeter readings is use of QFE altimetry. (see also What is wrong about this interpretation of QFE and QNH?)



    QFE altimetry which was used by some airlines (gone by now), and is still in use in Russia (undergoing phase-out), is where you set the altimeter's reference pressure so that it reads zero at some ground reference point, typically the runway threshold.



    So if the runway is above you, on terrain, you will fly at a negative height and your altimeters will show as such.






    share|improve this answer
























      up vote
      7
      down vote













      Besides flying below sea level, a situation for low and negative altimeter readings is use of QFE altimetry. (see also What is wrong about this interpretation of QFE and QNH?)



      QFE altimetry which was used by some airlines (gone by now), and is still in use in Russia (undergoing phase-out), is where you set the altimeter's reference pressure so that it reads zero at some ground reference point, typically the runway threshold.



      So if the runway is above you, on terrain, you will fly at a negative height and your altimeters will show as such.






      share|improve this answer






















        up vote
        7
        down vote










        up vote
        7
        down vote









        Besides flying below sea level, a situation for low and negative altimeter readings is use of QFE altimetry. (see also What is wrong about this interpretation of QFE and QNH?)



        QFE altimetry which was used by some airlines (gone by now), and is still in use in Russia (undergoing phase-out), is where you set the altimeter's reference pressure so that it reads zero at some ground reference point, typically the runway threshold.



        So if the runway is above you, on terrain, you will fly at a negative height and your altimeters will show as such.






        share|improve this answer












        Besides flying below sea level, a situation for low and negative altimeter readings is use of QFE altimetry. (see also What is wrong about this interpretation of QFE and QNH?)



        QFE altimetry which was used by some airlines (gone by now), and is still in use in Russia (undergoing phase-out), is where you set the altimeter's reference pressure so that it reads zero at some ground reference point, typically the runway threshold.



        So if the runway is above you, on terrain, you will fly at a negative height and your altimeters will show as such.







        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered 2 days ago









        user71659

        2,184617




        2,184617




















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