Can GPS measure airspeed?

The title is my question. Obviously it is in connection with the Lion Air crash which the Times article speculated was caused by a defective Pitot tube. (Amusingly the page 3 tickler called it a Pilot tube, but the main article got it right.)

I don’t see how it could, there’s nothing in a GPS unit that is measuring the speed of wind past the plane. How would you image this working?

Short answer, no.

GPS can measure groundspeed, but the airspeed is a measure of your speed relative to the air around you, which GPS would have no way to gather. For that, you would of course need your pitot tube.

Maybe the GPS speed measurement can be used as the pitot tube diagnostic. Say you are going at 30,000 ft and :

A. The previous airplane that flew that path reported a wind speed of 100 mph. You tally the GPS and pitot tube reading to get an idea if the pitot tube is functioning correctly.

B. You continuously plot the GPS speed and PITOT tube speed on a chart and if at a certain instance of time, the pitot tube starts showing low readings (i.e. the plots diverge suddenly), you know that the pitot tubes have been obstructed. (Unless the computer identifies another reason for this)

There has to be a pattern in which pitot tubes get choked/fail. Working in the chemical industry with hundreds of different sensors, we often see patterns in failure modes. An independent reading, however inaccurate but with modest repeatability (precision) helps in instrument health diagnosis.

ADS-B from the aircraft reported 300kt groundspeed after the initial climb. The exact airspeed is unknown, but with the aircraft at or below 5000 feet and no exceptional weather, it could not have been slow enough to stall. Something more happened.

I’m not an A&P tech, but AFAIK, pitot tubes are one of those things that just work unless they don’t. The failures I’m aware of are primarily human error like a plane fresh from the paint shop and someone forgot to take the masking tape off, or the tubes get blocked with ice because the pilot didn’t turn on the pitot heat. There have also been a handful of cases of wasps building nests inside the tubes.

I heard on the news last night that it was a malfunctioning stall sensor that was erroneously indicating a stall condition - and that this may have activated the stick pusher to force the plane into a descent so as to reduce AoA and recover airspeed. When functioning normally, the stick pusher system stops pushing the stick forward once AoA returns to an acceptable value, generally after a brief descent. This is helpful in an actual stall condition - not so helpful when there’s no actual stall condition and the system keeps the stick pushed forward until you crash into the ground. This is consistent with reports that the plane went into an unexpected dive early in the flight, and that the crash was at such a steep angle that the plane was shattered to bits on impact with the water.

In response to the crash, Boeing has re-issued a warning about how to safely deal with a malfunctioning stall sensor, presumably by (among other things) disabling the stick pusher.

Sorry, I forgot that it is airspeed, not ground speed that is relevant here.

This might be possible and has been discussed in some academic papers. Even without reliable pitot-driven airspeed, a lot is knowable about the aircraft orientation and flight path.

There’s a relationship between engine thrust, total drag and expected still air airspeed at a given altitude and attitude. In fact that relationship underlies the recommended manual procedure for unreliable airspeed indicators – use a certain pitch angle and throttle setting and at a given altitude the aircraft will fly at a certain approximate airspeed.

Engine thrust at a given throttle angle and rpm is known, likewise airframe drag at the given altitude. The altitude is known, either barometrically or via GPS, likewise outside air temp is presumably known since the only failure in this scenario is airspeed sensors. GPS can also calculate the rate of climb or descent. We also assume the multiply redundant attitude platforms are giving good pitch, roll and yaw data.

Given this data, the algorithm would compare the GPS groundspeed to the expected airspeed at the given thrust, altitude, attitude and flight path angle. If that algorithm indicated that all indicated airspeed sensors were drastically different from the GPS-calculated airspeed, in theory the flight control system as a contingency could switch over to calculated airspeed.

Since there are well-established pilot procedures for unreliable airspeed, I don’t know if this has ever been tested. I think it’s been considered for higher-end autonomous drones like Global Hawk, but I don’t know if has been implemented.

And at least one case of someone forgetting to remove the covers meant to keep the wasps out.

It wasn’t the stick pusher, it was the stabiliser automatically trimming nose down. The stick pusher can be overridden, the stab trim is incredibly powerful (it is the entire tail-plane, aka stabiliser, changing position) and full elevator is not enough to counter it.

Makes more sense. I knew the stick pusher could be overpowered by the pilot, but the only way I could see a crash occurring with this would be if the pilot(s) developed arm fatigue after fighting the stick pusher for a long time. (how strong is the stick pusher?)

Why would Boeing opt for (in the event of a stall warning) adjusting stabilizer trim instead of a stick pusher? Especially since counteracting the stabilizer trim would require a potentially esoteric and rarely used (and therefore easy to forget) procedure?

Just an anecdote, but I once took off in a Cessna 150 with a cracked pitot tube, resulting in the air speed indicator reading low. This was a very dangerous situation for me as a solo student pilot. I didn’t understand at first what was wrong, although it was immediately clear that something was wrong - I had been trained to climb out at a certain airspeed; in this case I was climbing out at a very shallow angle trying to maintain the indicated airspeed above stall speed, while worrying that I wouldn’t clear the tops of the trees a few hundred feet off the end of the runway.

What about weather though? I mean, if you’re flying into the teeth of a strong headwind, your airspeed and groundspeed are going to be pretty different without anything being wrong.

I read that the erratic altitude suggests they were having pitch problems. He’s trying to gain altitude , he’s thinking “go up to cruising altitude”, and “this is caused by poor speed control, I’ll just increase speed to be sure its above minimum”. When the pilot saw altitude dropping, he went for broke , and did things to get it back to climb.

But all the problems would be better explained by something strange going on back at the tail , and stalling by pitching up to much is just as bad a stall as turning off the engines and stalling.

The reason for pitot tubes is that the engine performance is related to the air pressure difference across the engine. So the thrust control is actually the setting of the requisite pressure difference. The confusion the pitot tube problem causes is that incorrect pressure info is getting to the engine thrust control, engine thrust control goes haywire, and while the pilot may think he increased thrust significantly by moving it from 2/3rds to 4/5ths, the pitot tube problem means he gets no change… because the controller is seeing bad pressure info.
The correct thing to do when the pitot tube problem arrises is to bump up past 100%. It knows then that the pilot wants a constant thrust of 100% thrust… and not a pitot tube controlled thrust.
So thats what I think he would do… set it to max, pitch the nose up and climb as hard as it can. This then pressured the tail to the max, and that is apparently when the tail was erratic/malfunctioning/broke/jammed or something.

It seems if he wasn’t predisposed to assuming it was pitot tube problem, he might have looked back at the tail, identified what was going on back there, and nursed it down.

Perhaps the problem was the pitot tubes though, and perhaps he assumed a setting of 100% thrust was going to open up the fuel throttle to 100%, and didnt utilise the proper way to bypass pressure control and set throttle to 100% … by pushing thrust up past 100% (or similar second step to say … emergency … give me 100% throttle and let me feel whatever thrust comes from that.)

Note that this issue is thought not to be the pitot tube, but the AoA (Angle of Attack) sensor.

While there is no claim that this was the only cause of the crash, disabling the stabilizer trim is the work around.

As AoA is newer, these pilots may not have known they had to disable it but that is a pure guess. As AoA is uniquely able to detect and correct for situations like accelerated stalls it is on during hand flying on these airplanes.

We will have to see what the final cause is, but it was not the pitot tube or the airspeed indicator from what is publicly known right now. An Angle of Attack indicator is completely separate from the pitot system or even IAS.

I should clarify, some AoA designs use two ports on the pitot to detect angle but on the 737 max 8 it is a vane sensor.

About 25 years ago my indicated airspeed went out. Worked fine until mid-flight. Had another pilot tell me to disconnect pitot tube from airspeed gauge and blow it out. It worked. Must have been a flying insect of some sorts. Needless to say without an airspeed indicator, I landed a little hot, just to make sure I was well above stall speed.

This was before gps was popular in planes. Although it wouldn’t have gave me my airspeed, the ground speed aspect through gps combined with me knowing roughly the wind speeds and direction with the windsock would have been helpful.