Are there any alternate instruments to indicate the airspeed in case of pitot tube failures?
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13$\begingroup$ Let me guess. They... wing it. $\endgroup$– ArtOfCodeCommented Jan 21, 2017 at 21:08
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4$\begingroup$ Pitch + Power = Performance. Know two variables and the other can be figured out. $\endgroup$– wbeard52Commented Jan 21, 2017 at 23:35
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3$\begingroup$ A known pitch trim setting should give you an approximate trimmed off speed though that changes with configuration changes. $\endgroup$– wbeard52Commented Jan 21, 2017 at 23:36
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7$\begingroup$ "What will the pilot do?" That depends on the pilot. In the case of the pilots on AF 447, they, stalled the aircraft and then crashed -- killing everyone on board. Perhaps you ought to ask "what should the pilot do" as that seems to be what you are curious about. $\endgroup$– KorvinStarmastCommented Jan 22, 2017 at 3:12
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$\begingroup$ @wbeard52 What you wrote was the core of an excellent answer. Why just leave a comment? $\endgroup$– KorvinStarmastCommented Jan 22, 2017 at 3:20
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5 Answers
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Some aircraft have angle of attack indicator, but most don't even though they have the sensor and use it for stall warning.
What you normally have is an unreliable airspeed procedure. This generally says that you should, depending on phase of flight, maintain specific pitch, set specified thrust and then adjust for rate of climb/descent while keeping the pitch.
Since aircraft is trimmed for speed, generally the procedure calls for being light on controls and correct mainly with engines. Exception is Airbus which keeps auto-trimming even with speed disagree warning on, so there side-stick controls climb and power controls pitch almost independently.
See also unreliable airspeed procedure for A330.
answered Jan 21, 2017 at 9:21
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Are there any alternate instruments to indicate the airspeed
From SKYbrary:
Reliable Sources of Information
The following information sources, independent of the pitot static systems, can provide reliable information for situational awareness:
rpm, and fuel flow, for engine thrust indication (not EPR, which may be unreliable);
Pitch and bank display;
FPV (Flight Path Vector) if available and derived from inertial and not barometric sources;
Radio height when below 2500ft agl;
EGPWS
Stick Shaker - may not always be activated but if it is, it is independently reliable;
Navigation systems can provide ground speed and position information (GPS can also provide altitude information);
Radio navigation aids and RNAV.
ATC, in a radar or ADS-B environment, can provide aircraft ground speed.
If TAS can be determined, a rough approximation of IAS at altitude can be calculated by the fomula: IAS=TAS – (FL÷2) eg 400TAS FL300 = 250IAS.
Note: Some aircraft systems are configured, as a safety measure, such that stick shakers and pushers will not operate if there is disagreement between systems. Thus, if the aircraft approaches and/or enters a stall, these safety features might not activate. However, if the stick shaker does activate, it should, in the absence of clear contrary indications, be believed.
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6$\begingroup$ I believe this answer can be improved by highlighting or explaining interesting portions, instead of quoting a large paragraph of text. $\endgroup$– kevinCommented Jan 21, 2017 at 14:11
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Depends of the type of aircraft - does it have a secondary pitot, inertial nav etc. If it does - you're in luck. Also - "Pitot failure" - that would mostly be clogged total pressure tube (insects . . . .), but could also be static, in which case pressure altitude & rate of climb may be unreliable too. In other words - have to watch for overall funny behavior.
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4$\begingroup$ Second pitot does not help, because you can't tell which one failed. You need third one. Intertial nav is useless, because ground speed and airspeed are related very loosely and you need airspeed. The only other instrument that can help is angle of attack, but while all large aircraft have AoA vanes, few have the actual reading shown in cockpit; usually it is only used for stall warning. $\endgroup$ Commented Jan 21, 2017 at 20:16
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In the planes I trained in, no.
In my training both for small planes and for gliders I did landings without instruments. You get a feel for the speed and height with experience. Of course a small GA plane or a glider are much simpler than a large airliner.
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An alternative is getting ATC involved. They could give an estimated speed of the aircraft. Since speed is distance travelled over time, ATC should be able to take the time difference between the two blips on the radar. Knowing the scale of the map in the radar, they can work out your speed.
For example:
If the radar has a scale of 1km represent 1cm on the radar screen, and the blip refreshes every 4 seconds. ATC could measure with a ruler (on the radar screen) the distance between the previous blip to the new blip, say this is 1cm. You know that 1 cm is actually 1 km in real life, and the the time between blips is 4 secs. So the plane has travelled 1km in 4 seconds which is about 250m per second (1000m / 4seconds). 250 m/s equals to about 900 kph. The ATC could give you updates on your speed.
