A farewell to pitot?
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None but a blockhead
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From: London, UK
A farewell to pitot?
There's an interesting article in New Scientist this week about a novel microphone technology. It involves tiny parallel strips of platinum electrically heated to around 200 Celsius, measuting the near-instantaneous cooling effect of impinging air molecules. The thermal inertia of the system is so low that it's possible to extract a lot of information about the direction and velocity of the air molecules - enough to distinguish conversations twenty metres away, for example, while processing out the unwanted noise.
It strikes me that this would be a very good technology for measuring air speed on aircraft, mostly because it should be very viable to deploy hundreds if not thousands of them per installation. You don't need very many failures in a pitot-tube based system to remove all useful data about airspeed (as we keep seeing), but if you could make the entire skin of an aircraft sensitive to velocity/pressure/temperature you could end up with a very much more robust system. I
We don't limit our personal environmental skin sensors to one or two patches on our body, after all. Comms/power/processing requirements for such a system won't be trivial, but we're very good at doing those robustly and cheaply these days.
It strikes me that this would be a very good technology for measuring air speed on aircraft, mostly because it should be very viable to deploy hundreds if not thousands of them per installation. You don't need very many failures in a pitot-tube based system to remove all useful data about airspeed (as we keep seeing), but if you could make the entire skin of an aircraft sensitive to velocity/pressure/temperature you could end up with a very much more robust system. I
We don't limit our personal environmental skin sensors to one or two patches on our body, after all. Comms/power/processing requirements for such a system won't be trivial, but we're very good at doing those robustly and cheaply these days.
Joined: Jun 2013
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From: Róisín Dubh
Apparently a similar system was developed decades ago, just measure the amount of electricity required to keep the plate at a set temperature and combine it with OAT and you've got your airspeed. Never saw the light of day because FAA and EASA specifically require aircraft to be fitted with pitot tubes to receive certification. Too expensive and complicated to have both. Beauracracy at it's finest
Joined: Jun 2013
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From: Róisín Dubh
Joined: Jun 2013
Aviation Qualifications: ATCO
Posts: 1,530
Likes: 101
From: Róisín Dubh
You may well be correct about EASA however, just something that was told to me back when I was a mechanic and I managed to set the probe covers on fire on an A340! Doh!
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From: France - mostly
.Una Due Tfc,
Thanks for providing that reference. I wasn't aware of that requirement, but thanks to PPRuNe I'm learning every day.
Just to be pedantic, your reference is an operating rule and not a certification requirement, and doesn't apply to airplanes operated under Part 121.
The operating rule obviously needs to be changed as soon as airplanes are certificated without pitots . However, that will not be soon. An airspeed indicator without pitots is highly unusual. Anyone who wants certification for such a system will have to provide evidence that it is at least as reliable as a more conventional system and meets the requirements for accuracy. It also needs to indicate airspeed adjusted for air density (CAS).
Thanks for providing that reference. I wasn't aware of that requirement, but thanks to PPRuNe I'm learning every day.
Just to be pedantic, your reference is an operating rule and not a certification requirement, and doesn't apply to airplanes operated under Part 121.
The operating rule obviously needs to be changed as soon as airplanes are certificated without pitots . However, that will not be soon. An airspeed indicator without pitots is highly unusual. Anyone who wants certification for such a system will have to provide evidence that it is at least as reliable as a more conventional system and meets the requirements for accuracy. It also needs to indicate airspeed adjusted for air density (CAS).
Joined: Jul 2013
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From: Everett, WA
I don't think you'll see Pitot probes go away any time soon. The alternate systems may show up as a backup or redundancy to the Pitots, but until there are millions of hours of operation demonstrating proper operation in every imaginable condition, no one is going to trust the airplane to them as the sole airspeed source.
Pitots are really not that hard to get right - unlike TAT probes, you can pump lots of heat into a Pitot probe since you're not concerned that the heat will corrupt the measurement (mud daubers remain an issue though
)
UDT - don't feel bad. I was on the first 747 Air Force about 25 years ago while they were doing some functional tests. Shop couldn't understand why one test wouldn't pass - I told them probe heat needed to be on, so they flipped the switch to turn it on. Shortly thereafter the probes were on fire
Pitots are really not that hard to get right - unlike TAT probes, you can pump lots of heat into a Pitot probe since you're not concerned that the heat will corrupt the measurement (mud daubers remain an issue though
)UDT - don't feel bad. I was on the first 747 Air Force about 25 years ago while they were doing some functional tests. Shop couldn't understand why one test wouldn't pass - I told them probe heat needed to be on, so they flipped the switch to turn it on. Shortly thereafter the probes were on fire
Last edited by tdracer; 7th October 2013 at 18:30.
