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About a week ago, I was flying from Boston to San Juan in an A320 and noticed that the flight was staying at about 42000 feet for the major part of the trip, according to the flight map screen on the plane's entertainment system. That struck me as an unusually high altitude and I am even more surprised now that I have read a little on the subject. I am wondering if that was even possible, or the altitude readout to the entertainment system was simply wrong. The flight proceeded uneventfully in other ways, and was on time.

What is the ceiling altitude for the Airbus A320?

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    $\begingroup$ According to wikipedia the service ceiling for A320 is 39,000 ft (the A319 and A321 have a service ceiling of 41,000 ft). $\endgroup$
    – Jeff B
    Apr 22, 2016 at 5:30
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    $\begingroup$ Do you remember the flight number and exact date? $\endgroup$
    – anshabhi
    Apr 22, 2016 at 6:53
  • $\begingroup$ From a legal standpoint in the US: Flying above 40,000 ft requires special certification, but is not impossible for a civilian aircraft (many business jets fly well above). This is because in case of depressurization the aircraft must go down to 15,000 or 10,000 ft to find safe breathable air and this takes time, but hypoxia can create permanent brain damages or be lethal in a few seconds. See US 14 CFR 25.841. At a previous time the ceiling was 41,000 ft. $\endgroup$
    – mins
    Apr 22, 2016 at 7:38
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    $\begingroup$ Was this pressure altitude or geometrical height? The certified service ceiling is based on pressure altitude, the geometrical height may be higher, especially in high pressure area's and warm weather conditions. $\endgroup$
    – DeltaLima
    Apr 22, 2016 at 11:33
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    $\begingroup$ I have just found the flight stats for my flight -- it departed minutes into April 16, so I was not seeing it listed for April 15th. I am sorry, this probably was a non-issue from the start. The altitude is listed at more or less steady 36000 feet throughout.. Also, I just noticed that the LATITUDE reading is in the vicinity of 42... "latitude" vs "altitude" -- could be easily confused (just guessing). In any case, it seems as though my impression of the altitude was not correct. Thank you for all the answers. $\endgroup$
    – deercat02
    Apr 23, 2016 at 4:33

3 Answers 3

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This question is a bit more generic. Ceiling altitude depends on many things such as (environmental conditions, engines operative, a/c aerodynamic characteristics, e.t.c), each factor by itself and in combination, can affect the ceiling altitude. You can get an idea and some definition from here: https://en.wikipedia.org/wiki/Ceiling_(aeronautics) .

For most of the A319/A320/A321 aircrafts Maximum Certified Altitude according to FCOM is FL391 considering (2 engines operative and environmental allowing.)

Now according to FCOM DSC-22_20-50-10-28, (controls and indications) you can find the MCDU page,

REC MAX: This field displays the recommended maximum altitude (in magenta), that is computed based on the current gross weight and temperature, and assuming that the anti-ice is off (Refer to QRH graphs, if icing conditions are expected). It provides the aircraft with a 0.3 g buffet margin, a minimum rate of climb at MAX CL thrust, and level flight at MAX CRZ thrust. This field is limited to FL 398. If one engine is out, this field displays the recommended maximum engine-out altitude, that is computed based on the long-range cruise speed and assuming that anti-ice is off.

enter image description here

Note: The OPT or REC MAX field may display a value up to FL 398. However, the current aircraft’s maximum certified altitude is 39 100 ft. Disregard the OPT or REC MAX value, if it exceeds FL 391.

Now for entertainment system, (again according to the books), reads and display data taken from the FMGC.

One scenario is the input from the pilots, in MCDU was the displayed LVL, and subsequently in the entertainment system, the displayed LVL was that (after the CRUISE phase), even though the actual altitude was different that the displayed.But again it's impossible to insert an altitude above the value of "FL398" MCDU says "ENTRY OUT OF RANGE".

So the conclusion As kevin said already, possibly wrong data were displayed.

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    $\begingroup$ Hello Lidakis Emmanuel, welcome to Aviation.SE! $\endgroup$
    – DeltaLima
    Oct 18, 2016 at 15:08
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Let us state the known facts (from the question and its comments):

  1. The altitude reported on the passenger entertainment system was observed to be 42,000 feet.
  2. The aircraft was an Airbus A320.
  3. Looking up the flight history shows the plane flown at 36,000 feet.

Case #1: aircraft was flying at FL420

As noted in the comments, this is beyond the certified service ceiling of the A320. Also, this would contradict (3), which data is collected via ADS-B and presumably correct.

Case #2: the OP misread "latitude" as "altitude"

From my experience flying as a passenger, the raw longitude and latitude data is not displayed on the passenger's flight information screen. There would be no need to, since (most) passengers would not carry a map with them and plot the coordinates onto it. The coordinates themselves are therefore useless information, and I doubt the engineers who designed the software of the entertainment system would put it in.

Case #3: reported altitude was geometric height

I don't have a source for this, but I don't think the passenger's entertainment system would have a function to convert pressure altitude into geometric height, when even the pilots don't have it.

Case #4: the reported altitude does not correspond to the altimeter's reading

The flight information display reports data captured in the cockpit in real time. Data such as the plane's location, speed, altitude, vertical speed, heading and winds aloft, are supplied by the flight management computer and are identical to those seen by pilots. I know this is the case as I always watch the flight information screen closely during takeoffs and landings, and the readings correspond to how a pilot would have flown a SID or STAR. (I always fear the pilots may mess up and I would feel safer if I'm in the cockpit, but since I'm not allowed to do that, I watch the flight information screen to act as "passenger monitoring".)

I'm left with one conclusion:

The altimeter's reading was wrong. There are multiple altimeters on an airplane like this. If one of them malfunctions, the pilots can set the source of the autopilot and flight management computer to another good altimeter. But since the passenger's flight information display is a non-critical system, its source probably cannot be switched, hence the wrong data is displayed in the cabin (most passengers don't know what are the nominal values anyway).

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    $\begingroup$ Ad case #3: It is plausible and likely. The pilots have no use for geometric altitude, so it is not provided to them, but it is readily available from GPS and used at least by EGPWS. So IFE system should have no trouble getting it. And it is likely to use it, because it is what laymen expect. $\endgroup$
    – Jan Hudec
    Apr 23, 2016 at 20:17
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    $\begingroup$ That said, having pressure altitude 36000 correspond to geometric altitude 42000 would require very high pressure or very high temperature. $\endgroup$
    – Jan Hudec
    Apr 23, 2016 at 20:25
  • $\begingroup$ Thank you very much for looking at the problem from all possible angles. I agree that your Case #4 is the only plausible explanation, conspiracy theories aside. The altimeter feeding data to the entertainment screens must have been flawed. Flying the same route home, I was observing a steady 36000 feet reading. $\endgroup$
    – deercat02
    Apr 23, 2016 at 22:52
  • $\begingroup$ Case #3 seems unlikely. The question says the altitude was 42,000ft for "a major part of the trip" and, while Boston is indeed at about 42deg north, San Juan, Puerto Rico (the only San Juan that currently has flights to Boston, and these are by JetBlue, which flies Airbuses) is at 18deg north, due SSE of Boston. On the other hand, some passenger flight information displays do give the latitude and longitude, contrary to your suggestion that they wouldn't. I've often noted down the co-ordinates when taking photographs through the window. $\endgroup$
    – David Richerby
    Oct 22, 2016 at 12:23
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Nobody ever flies at FL420, ever. RVSM airspace, where usable altitudes are 1000' (10 FL's) apart, stops at FL410. Above that, only "odd" altitudes are used: FL430, FL450, FL470. Between the lower service ceiling & the unusability of FL420, I'd totally agree that the entertainment system's altitude readout was wrong. What causes this, however, is a harder question; if the aircraft altimetry systems were messed up, getting an ATC clearance into RVSM airspace (above FL280) could be problematic at best.

I'd wonder if the entertainment system wasn't jumbling things around somehow & putting bad data on the displays. And then that glitch was fixed in the "return home" airplane.

(Side note - @JanHudec makes 2 excellent point in comments on the other answer about the possibility of the IFE using GPS altitude. Other then the magnitude of the 6000' delta between the flown altitude and the reported altitude, I could readily believe that the IFE might be reporting GPS altitude rather than the more accurate info from the FMS.)

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  • $\begingroup$ It has just come to mind that the altitude reading on the entertainment screen that one typically sees in flight is usually remarkably steady and looks, maybe, like a rounded number(?). I am wondering if the airplane is actually staying rock steady at exactly FL370, for example, or the readout is "stabilized" and rounded off before it reaches the entertainment screen. If that were the case, the "math" of that process could have been the culprit. I wonder if the pilots see a more dynamic digital real time altitude readout in the cockpit. $\endgroup$
    – deercat02
    Apr 24, 2016 at 2:46
  • $\begingroup$ I need to look up to verify this, but if the plane has 3 static ports (which is a reasonable assumption), having one of them inoperable may not ground the plane or keep it out of RVSM according to the MEL (minimum equipment list), as long as the pilots are aware of it and adjusts the source correctly. $\endgroup$
    – kevin
    Apr 24, 2016 at 8:33
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    $\begingroup$ I've been cleared FL410B430 so it isn't strictly correct to say that no one ever flies at FL420. In this case, however, they certainly weren't. $\endgroup$
    – sbooth
    Apr 24, 2016 at 12:23

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