Death of Pitot system

Many systems we have used for years and assumed were "indispensable" are slowly being replaced. Their demise is often characterized by cantankerous pilots as the decline of aviation, the beginning of the end, or even as unsafe (sometimes this is true). We have seen vacuum systems be completely replaced by robust electrical systems, VOR's replaced by GPS, etc.

Many of us think pitot systems are irreplaceable because, well, we "need" airspeed, right? Well I was thinking last night about this. I fly some great machines bought and paid for, as well as maintained by, all of you wonderful taxpayers (thank you). This is to say that if I have ANY gripe, I just write it up and it gets fixed. Well, even with this level of maintenance, we often have a few knots of slop between sides and between reality in the pitot systems (2 independent systems for each side, with the ability to switch between sources, all glass panel). One system we always thought was "required" was a doppler radar for use while hovering at low altitudes over water (If you think this is not necessary, try hovering a helicopter at 70ft, at night with no visible horizon, over the same spot without getting vertigo or drifting due to the rotor wash in the ocean!). We now just use GPS position coupled with INS...NO doppler!

I see a possibility in the not too distant future where we have accurate and reliable ADS-B info (ground winds for every station as well as regional/altitude enroute data). If this data can be made accurate enough (might already be), vector summed with your GPS ground vector, voila, "airspeed". Sure, the numbers would not be perfect, but I am thinking back to every time I have compared my IAS(pitot) with my computer generated AS (GS summed with winds) and it is almost always within a few knots...the slop I already have in my pitot system. So, if this would be reliable enough, you could get rid of the pitot tubes, static ports, alt air source, included tubing, electronic sending unit, POWER for heated pitot, aerodynamically cleaner aircraft...

Thoughts?

That might work - right up until you throw Mother Nature in the mix with convective activity. The time lag between the ground-based projection of the wind shear on short final and your physical interception of it (not to mention dynamic weather in cruise) will produce lots of wrinkled metal and happy lawyers.

I predict the reports of the pitot tubes death may be greatly exaggerated (apologies to Mark Twain).

Interesting thoughts and logic about the impending death of the pitot. Personally, I would be surprised if we still measure airspeed almost exclusively with a pitot in 50 years but I think the demise is not going to happen in the immediate future.

My knee jerk reaction is a GPS derived airspeed that incorporates winds aloft is practical today. I think most EFISs could be programmed to do that now. The pitot system is fairly simple, robust and independent of microprocessor failure, government degraded GPS signals (which happens near me too often) and inaccurate or delayed winds aloft speeds. It seems to me there's wisdom in adding the derived airspeed value but a risk in removing the pitot system. The risk probably isn't immediate or constant, which makes it insidious. Perhaps 5%, 10% or maybe even more, your airspeed could be off by 20% or more. It seems to me that's cutting it close on an approach speed, and could be hazardous with Vne and Va speeds.

Having said this, I expect the improved reliability of the current systems noted above, the expanded use of AOA indicators, and perhaps the development of an onboard electronic (read reliable and leakproof) way of directly reading airspeed will eventually lead to the demise of the pitot. I'd be looking 20-25 years out for experimental aircraft and probably July 1, 2150 at the earliest for certificated aircraft. By that time heated pitot tubes should be selling for about the same cost as a wing kit today.

I think real-time, local measurement of the air pressure acting on the aircraft will always be useful. Direct measurement of ram air pressure is important for staying within airframe limitations and preventing stalls, etc. i agree that using IAS for navigation calculations is anachronistic, though. apart from Vne/Vno we're really most concerned about groundspeed (which gps does better) and AOA. "approach speeds" are just a proxy for AOA that varies with loading, density altitude, etc. Locally measured AOA is still always gonna be useful.

What's really anachronistic though is using pressure gauges which have to be constantly recalibrated (on an hourly basis or more) as the primary means to determine altitude. This is especially true in the flight levels, where the absolute pressure differences between altitudes are tiny. It can cost upwards of $100K to retrofit a legacy turbine aircraft with better altimeters (super accurate pressure gauges) to qualify for RVSM. WAAS GPS can locate an aicraft within a handful of meters even vertically. we ought to all be flying GPS altitudes, and occasionally adjusting our altimeters to match the gps so we have a backup in case of gps failure. Flying pressure altitudes around traffic should be an emergency backup procedure.

Backup system still nice to have

I can see the possibility of a GPS system eventually displacing the Pitot/Static for primary references, but I do not see the P/S going away, it is pretty standard practice to have a backup system that is not driven by the same source as a primary------in this case electrical vs just plain old air.

LAMPSguy said:

....We have seen vacuum systems be completely replaced by robust electrical systems, VOR's replaced by GPS, etc.....

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One thing I never understood was why didn't VOR receivers ever get married to the nav computers that started becoming popular back with the early Loran sets?

LAMPSguy said:

....I am thinking back to every time I have compared my IAS(pitot) with my computer generated AS (GS summed with winds) and it is almost always within a few knots...the slop I already have in my pitot system. So, if this would be reliable enough, you could get rid of the pitot tubes, static ports, alt air source, included tubing, electronic sending unit, POWER for heated pitot, aerodynamically cleaner aircraft....

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So where would the wind data come from?

Dave

flyeyes said:

I think real-time, local measurement of the air pressure acting on the aircraft will always be useful.

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I thought by now, someone really smart would have come up with a way to measure air molecule passage or pressure with something like a strain gauge to replace the pitot static system. Some magic electrogadget that measures how many air molecules pass to calculate airspeed or a change in pressure for AOA.

David Paule said:

One thing I never understood was why didn't VOR receivers ever get married to the nav computers that started becoming popular back with the early Loran sets?

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Actually, they did. FMS systems used VOR, GPS, LORAN, and even NDB inputs. Operationally most of this was in the background, the system utilizing the best information. I used to fly a Collins system that not only automatically used VORs, it would identify the station and display the ID.

As much as I like the modern systems, being an "old guy," I still like to have a simple backup. Pitot static, like a whiskey compass, is simple backup, not dependent on other aircraft systems.

John Clark ATP, CFI
FAAST Team Representative
EAA Flight Advisor
RV8 N18U "Sunshine"
KSBA

A few thoughts.
Several posters are confusing IAS and TAS. But knowing one this proposed box could calculate the other.
Winds aloft are currently forecast, not measured. I see this as the biggest problem. Run into some strong turbulence and want to slow to Va (which is an IAS)? This scheme requires good knowledge of the wind.
Using GPS (true) altitude makes a lot of sense to me. But true altitude is not the same as barometrically determined altitude (indicated altitude) so everyone, even casual vfr fliers, would need to convert. Interesting that precision gps approaches (LPV) still use a barometric altimeter to determine DA, not the gps altitude.

I can see an AOA system taking the place of pitot/static for low altitude and pattern uses, and a GPS for enroute guesstimates, perhaps. An AOA system would serve all the major purposes of the pitot-static in the case of sudden wind shifts and stall awareness - but then the question becomes how do you get AOA without pitot/static? Pressure differential across several ports on the leading edge? That can quickly become more technologically difficult and expensive than a simple pitot tube.

Timeframe

All certainly valid points, I was certainly not advocating ripping out the systems now!

As for molecular counting, there are very similar systems on submarines. Not sure exactly how they work...probably know salinity, depth, temp, calculate density and can therefore make a pretty accurate guess as to the speed.

IAS vs TAS vs CAS...numbers that I think get used interchangeably by so many people in so many situations.

Just exercising the brain with this one.

Engineering advancements occur when old engineers (like me...) retire. ("Why, in my day we had 6 pitot tubes and we LIKED it!")

Solid state sensors reliably detect the movement of wheels and other things in our cars - at a couple of dollars or less per sensor. You would think they could come up with a similar sensor that could measure the movement of the air itself some small distance off of the fuselage (then calibrate that). Something that would look like a little blade antenna or even a OAT probe.

The whole "static system" thing also seems silly nowadays. A flush-mount pressure sensor ought to have that value (ambient pressure) electrically as data, with no static system plumbing. Hmmmm - same thing in the pitot for that matter - the pitot tube becomes a dead end tube to a pressure sensor. Same for the AOA - after all, those three tubing lines are just running to a computer box (ADAHRS) with those exact sensors and conversions in it!

EDIT: SORRY LARRY - MISSED YOUR POST ABOVE! Maybe we have a business here...

my prediction would be that we never move away from actual realtime air speed measurements. everything else remains an inferior backup.
however, the current way of doing it with long tubes being split into several parallel measurement units with possible leaks and hysteresis is quite outdated. also, the suceptibility to ice, bees (and forgotten pitot covers) will be further reduced.

as everything becomes more integrated, it is about time for someone to come up with electronic pitot respectively static units that digitize right on the spot and distribute the data electronically.

also, solid state measuring methods using laser doppler or ultrasound might one day replace the traditional pressure sensing.

as for altitude, pressure sensing will likely remain prime for a long time. both due to legacy compatibility issues as well as its connection to performance/aerodynamics. but who knows, maybe some day it will be easier and more accurate to volumetrically count molecules ;-)

regards,
bernie

edit: several posters having the same ideas while writing the post ;-)

Just to give you a data point from a fellow helicopter guy, our aircraft technical manual specifically states 'a failure of the air data computer does not constitute a grounding condition'. The pitot and static data are fed to the air data computer. That data is then combined with INS and GPS data to give us airspeed, velocity vectors, wind, etc. Essentially a loss of the pitot static system does not ground the aircraft. We are still required to have backup pitot static driven standby instruments that work, but these aren't used unless everything else is lost. I flew one the other day, high speed low level with a failed air data computer. There is no noticeable difference at all, except we do not have in cockpit wind information. I could easily do without a pitot static system. I think the problem is going to be when you lose all electrical equipment. Pitot static is all you will have left.

Bill_H said:

The whole "static system" thing also seems silly nowadays. A flush-mount pressure sensor ought to have that value (ambient pressure) electrically as data, with no static system plumbing. Hmmmm - same thing in the pitot for that matter - the pitot tube becomes a dead end tube to a pressure sensor. Same for the AOA - after all, those three tubing lines are just running to a computer box (ADAHRS) with those exact sensors and conversions in it!

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These are already in service... there's basically an air data computer built into the base of a pitot tube.

http://www.goodrich.com/Goodrich/Bu...ucts-and-Systems/SmartProbe™-Air-Data-Systems

We could be using a pressure sensor right in the pitot head or a solid state device to measure the temperature rise and then just run the data to our boxes.
It would save a bunch of plumbing.

There are many other ways of measuring air flow. Hotwire mass airflow sensors, ultrasonic Doppler shift, etc. All have trade offs.

flyvans.com said:

my prediction would be that we never move away from actual realtime air speed measurements. everything else remains an inferior backup.

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This. The only number that matters when it comes to the safe operation of an airplane is local air speed over the wing, right now. The pitot tube is just too simple, too cheap, and too reliable to be replaced by some kind of GPS derived wizardry.

It would be akin to having some kind of off board system telling you what your rotor RPM is... how comfortable would you be with that?