Reliable airspeed measurment

[atakacs]

It would seems that one of the Achilles' heel of modern FBW airliners is the lack of reliable airspeed measurement. Off hand I can think of obvious AF447 (https://en.wikipedia.org/wiki/Air_France_Flight_447), the XL Airways A320 (https://en.wikipedia.org/wiki/XL_Airways_Germany_Flight_888T) crash or apparently the very recent AN-148 (https://www.pprune.org/rumours-news/605282-148-missing-after-takeoff-moscow-7.html#post10060429) crash.

From that latest tread I quote

Laser based speed sensors are in R&D phase, Airbus have done trials. Cost is an issue, so defense applications are likely to be first customers.

Really a discussion for another thread.

Well I'm posting here because I think this seems to be a major items that should be solved industry wide.

And regardless of the measurement itself I think that there is room for improvement in both the detection of the issue and the help provided to the crew by the automation (or lack of, as things can get rather confusing when a FBW system is presented with what should be detected as obvious bogus data).

Thoughts ?

[Jimbo2Papa]

I don't consider reliability of airspeed measurement to be more or less an achilles heel in FBW aircraft compared to non-FBW.
What appears to be the achilles heel - in those examples you mention - is the training that is done to recognise, accurately diagnose, and then deal with the upset.

[Piltdown Man]

But why do we need an airspeed indication? Alpha is the best value to have and that is the one we are denied. It’s also pretty cheap to stick an alpha value display. The Achilles heel is the systems design that robs us of critical information.

[atakacs]

I would agree to a point. However I'd venture to say that on modern FBW aircraft things can get rapidly out of hand in such circumstances where a lower tech plane will just keep flying 🤔.
Of course proper training / procedures are paramount.

[underfire]

In reality, I really dont think the lasers are a viable method, far too many issues, even in clear air, let alone fog or clouds.

[PDR1]

I would agree to a point. However I'd venture to say that on modern FBW aircraft things can get rapidly out of hand in such circumstances where a lower tech plane will just keep flying ��.


I'm not sure that stands much scrutiny - blocked/frozen pitot-statics have caused aeroplanes to rapidly get away from their crew for decades. Nothing to do with FBW. For examples took at Aeroperú Flight 603, Birgenair Flight 301 or many others...

[DaveReidUK]

l@ser based speed sensors are in R&D phase, Airbus have done trials.Not just Airbus.

BAE Systems develops laser airspeed sensor for aircraft (https://www.baesystems.com/en/article/bae-systems-develops-laser-airspeed-sensor-for-aircraft)

[Chesty Morgan]

Piltdown Man

Can’t imagine ATC streaming would be particularly effective if we all had to fly specific alpha! ;)

[Bergerie1]

Dave,

That BAE LASSI system looks really interesting but, surely it can only measure TAS whereas what is needed is IAS. How is this resolved?

[DaveReidUK]

Even if you have no air data at all (dynamic or static), GPS will give altitude sufficiently accurately to be able to be used in the standard TAS -> IAS calculation.

[victorpapa]

On the B787 if ADRS info is invalid it switches to AOA speed and GPS altitude. PFM :)

[DaveReidUK]

GPS will give you accurate track and ground speed.
The calculated wind is solved using track & ground speed, heading and TAS. TAS is calculated using IAS and air density. (If IAS is rubbish, then TAS is rubbish).

Therefore TAS-> IAS calculation will be rubbish.

I don't understand what point you are trying to make.

TAS is calculated using IAS and air densityCorrect.

And the corollary is that if the requirement (as in this case) is the opposite way round, i.e. to calculate IAS from TAS, you get TAS from the laser sensor (which is what this thread is about) and air density from altitude (either baro- or GPS-derived).

What's your problem with that?

[Goldenrivett]

Hi Dave,
Sorry - Please disregard my post.
I thought you meant we could calculate it now using the displayed TAS.

I'll delete it.

[clark y]

Maybe a low tech solution would be to scatter the sensors around the aircraft hull a bit more. Everything seems to be bunched around the nose on most aircraft. Why not a pitot up the fin or out on a wing. Same with AOA sensors.

[vapilot2004]

The most accurate pressure measurements are going to be away from any disturbances in the air stream caused by the aircraft, which begin at the nose and widen out from there. Some of these disturbances change and move about depending on the aircraft's flight path, yaw moment, the wing's AOA, flight control position, and so on.

While it is true that there are other pitot and/or static probes and ports in various places around the aircraft; the engines, yaw damper, and rudder limiter, for example, these positions have different precision requirements than those in the nose, and any local disturbances are accounted for in the signal use and conditioning.