So my boss gave me a question today as my daily quiz: Let's say an A321 is cruising on a 6-hour flight. It reaches FL350 30 minutes after take-off. After another 8 minutes there's a discrepancy between 2 altimeters. PFD 1 reads 35000 ft while PFD 2 reads 34500 ft. The pilot decides to make an air-turn back due to insufficient fuel, despite having enough fuel filled while on ground.

The question is, did the pilot make the right decision?

I've done some research. The difference between 2 altimeters is not supposed to exceed 245 ft at 35000 ft. But I don't know how that is related to fuel shortage. Worst case scenario the static port is contaminated.

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    $\begingroup$ Hello Light Le, welcome to Aviation.SE. Nice quiz question, if your boss has more of those, keep us informed :-) $\endgroup$
    – DeltaLima
    Commented Sep 19, 2016 at 11:04
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    $\begingroup$ @DeltaLima sure I will. My boss loves to grill me on these kinds of trivia and won't leave me in peace until he gets a satisfactory answer (his direct quote). $\endgroup$
    – Light Le
    Commented Sep 21, 2016 at 1:23

1 Answer 1


If the aircraft is cruising at FL350 it is operating in Reduced Vertical Separation Minima (RVSM) airspace (between FL290 and FL410).

With an altimetry discrepancy between the two altimeters of more than 245ft, the aircraft is no longer RVSM approved. It shall therefore descent or climb out of the RVSM altitude block.

For the Airbus A321 the operating ceiling is below FL410, so the only option is to descent to FL280 or below. Since the fuel consumption in those altitudes is significantly higher the aircraft is unlikely to complete its 6 hour flight with the amount of fuel on board. Therefore the decision to return is likely the right one.

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    $\begingroup$ Awesome :-) QED $\endgroup$
    – mins
    Commented Sep 19, 2016 at 10:49
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    $\begingroup$ Excellent :) I'm not an aviator, but this style of question is great for technical and procedural testing in interviews $\endgroup$ Commented Sep 19, 2016 at 12:08
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    $\begingroup$ @Nelson make it as a separate site question. "What happens if a pilot flies in RVSM airspace with a craft not RVSM approved?" $\endgroup$ Commented Sep 19, 2016 at 14:53
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    $\begingroup$ @Nelson - If the discrepancy is known outside the cockpit, the pilot could be grounded for this kind of infraction in a commercial airliner. RVSM essentially means 1000 ft vertical separation between aircraft. If your altimeter is off by 250 feet, and the other guy's altimeter is off by 250 feet the other way, now you're passing within 500 ft of another plane even if you're keeping the needle right on your flight level; that is generally the absolute minimum safety margin for flight. Deviate even a little further and now it's a "near miss". $\endgroup$
    – KeithS
    Commented Sep 19, 2016 at 14:55
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    $\begingroup$ If he's flying in RVSM space outside the limits of altimeter discrepancies, then facing administrative action is literally the BEST possible outcome. A head-on collision with another plane, resulting in mass casualties is an entirely realistic possible outcome. He won't worry about getting arrested after that. $\endgroup$
    – abelenky
    Commented Sep 19, 2016 at 14:58

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