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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    To cruise at low altitude in Death Valley? – quiet flyer Nov 8 at 18:44
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    @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 Nov 8 at 18:57
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    Yes. me too, that's why I didn't form it into a real answer. Good question. – quiet flyer Nov 8 at 18:59
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    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 Nov 8 at 19:14
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    @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 Nov 8 at 19:36
up vote 39 down vote accepted

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).

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    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 Nov 9 at 1:01
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    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 Nov 9 at 1:15
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    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 Nov 9 at 14:51
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    @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 Nov 10 at 5:23
  • 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 Nov 10 at 6:13

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.

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    For certain values of "hilarity." – Zach Lipton Nov 8 at 23:40
  • Nice answer. Thanks. – Darren Nov 9 at 0:49
  • "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 Nov 9 at 6:00
  • "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 Nov 10 at 6:17
  • @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 Nov 10 at 16:06

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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