30

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?

user71659
  • 6,606
  • 1
  • 21
  • 35
Darren
  • 605
  • 6
  • 8
  • 37
    To cruise at low altitude in Death Valley? – quiet flyer Nov 08 '18 at 18:44
  • 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 Nov 08 '18 at 18:57
  • 2
    Yes. me too, that's why I didn't form it into a real answer. Good question. – quiet flyer Nov 08 '18 at 18:59
  • 5
    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 08 '18 at 19:14
  • 2
    @ChrisLau a Dash-8 Q400. – Darren Nov 08 '18 at 19:15
  • 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). –  Nov 08 '18 at 19:36
  • @757toga yes you are right. On Boeing and Airbus, you will have a compilation of egpws, rad alt, and ILS Location and GS. So it is targeting a trajectory and will do the flare when 50ft agl is reached. – Chris Lau Nov 08 '18 at 19:40
  • Now for that story. If you are flying at LFBO when the altitude of the threshold on 32L is 512ft and your FCU is targeting 500ft. Your aircraft will go to the ground. The non precision of asl and agl makes that statement on targeting 0ft not consistent. – Chris Lau Nov 08 '18 at 19:41
  • 2
    @ChrisLau - the OP is basically wondering why the altitude selector could be set to "0" (wording in the story is a bit confusing). Anyway, as you know, you can rotate the knob and set in 0 or anything above that. It is used for reasons beyond interacting with the autopilot (normal FD altitude capture, etc.) . Although, I can never remember actually setting it (altitude selector) to "0," it is physically possible. –  Nov 08 '18 at 19:55
  • @757toga agreed. – Chris Lau Nov 08 '18 at 19:58
  • 2
    The full AAIB report is here: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/749117/Bulletin_11-2018_Hi_Res.pdf - the news story is a bit simplified, and I think it's wrong that the setting happened after takeoff, the main trigger is described in the section "Incorrect setting of the FD guidance modes prior to takeoff" – GS - Apologise to Monica Nov 09 '18 at 08:04
  • 1
    Why would a descent of 500 ft in 18 seconds be considered a "drop"? That's a bit over 1600 FPM: a steep descent, but hardly a drop. – jamesqf Nov 09 '18 at 18:18
  • @quietflyer Or in the Jordan River valley, parts of which are over 1,000 feet below sea level. – reirab Nov 10 '18 at 06:04

3 Answers3

39

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).

Mark Omo
  • 105
  • 4
M28
  • 2,459
  • 12
  • 28
  • 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 Nov 09 '18 at 01:01
  • 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 Nov 09 '18 at 01:15
  • 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. – M28 Nov 09 '18 at 14:51
  • 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 Nov 10 '18 at 05:23
  • 1
    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 '18 at 06:13
12

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.

John K
  • 130,987
  • 11
  • 286
  • 467
  • 11
    For certain values of "hilarity." – Zach Lipton Nov 08 '18 at 23:40
  • Nice answer. Thanks. – Darren Nov 09 '18 at 00:49
  • "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 09 '18 at 06:00
  • 2
    "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 '18 at 06:17
  • @JohnK: The phrase may have been popularized by Frasier, but it is far older than that. Here's an example from 1872: https://books.google.com/books?id=1nBgZNraDdkC&pg=PA317&dq=%22hilarity+ensues%22&hl=en&sa=X&ved=0ahUKEwjGp_6HmsreAhUT7Z8KHfBQDLsQ6AEINDAC#v=onepage&q=%22hilarity%20ensues%22&f=false – Eric Lippert Nov 10 '18 at 16:06
  • @reirab well yeah I could also dig a mile deep pit and put an airport there, but we are talking about the normal world here, not outliers like that. – John K Nov 11 '18 at 01:41
7

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.

user71659
  • 6,606
  • 1
  • 21
  • 35