Why use only pitot-static system for altimeter/airspeed
 

Hello all,

Yesterday I was watching AF 447 documentary on natgeo. What crossed my mind:
Why there is no backup system if all pitot systems fail - i.e. radio altimeter or at least GPS. I know this is not too precise, but it is better than nothing or even worse, false indications?
Other words, in 21st century, with computerized airplane, we still rely fully on "hole-and-pipe" technology, with no alterantive backup?

Radio altimeter is not working at that altitude
It is so inprecise, that it is basically unusable. The difference between (GPS) groundspeed and airspeed can be much larger than the speed margin between stall and overspeed.
For good reasons we do have an alternative for about 100 Yers, it is called pitch and power. But mayby some people have forgotten it since...

Well said Volume. I couldn't have put it better myself.

Thanks for the prompt answer.

Regarding radio-altimeter not working on cruise altitudes, I presume it is because of the time it needs for the beam to go and return back from earth to the airplane which has already "gone" forward?

Regarding altitude what about GPS as alternative? In AF447 they had a false reading that airplane is descending, so copilot puiled up and that is how the trouble started. Although GPS is not too precise reading altitude, wouldn't it at least give a "hint" that they are not descending?

AFAIK, AF447 had altitude read-outs at all times- it was airspeed that was in error.

I do agree that a read out of GPS altitude might be useful in some circumstances- more so that Rad Alt, as that is actually a reading of height above terrain, not altitude.

Indeed, the smart landing "Altimeter Setting" system works by comparing Baro to GPS altitude.

Similarly, the AF flight WOULD have had a ground-speed reading from it's inertial system- which includes GPS. but ground-speed, as has been pointed out, is of little use as a flight instrument at altitude, and without an air data source, there is no way to calculate IAS or TAS.

Some Airbus aircrafts are fitted with the BUSS, you're flying with the AOA and GPS altitude (even if not precise, it's still showing the altitude change trend and is still better than nothing). It becomes active as soon as the 3 ADR's are turned off.

Some alternative to pitot-static system for airspeed indication
 

One Australian professor has suggested an alternative system using laser beam and Doppler principle to find airspeed as this youtube video shows: Its reliability,accuracy and certification by Aviation authorities is not shown in the video.

The pitot-static system tells you the local airspeed, which is crucial because you fly in the air around the aircraft, not with respect to the ground.

Some sort of calculated back up might be helpful, but if the temperature or wind changes, the old value won't be the right one to maintain.

A GPS would have told AF447 that they were plummeting to the ocean and their groundspeed was way down from its cruise value, but they should have recognized that already from the altimeter. If there were skeptical of the altimeter, then the GPS might have provided a reference to start to piece things together, not that things should ever have been in that state in the first place.

well, it is about the much higher power you need to receive back an echo from 30.000 ft away, and you will have too much scatter from all the different hills on the ground reflecting.

LDA is a great ide, but it works only because there are particles in the air. I am afraid it is too clean up there, to really work. Otherwise, it is absolutely perfect, as it is a truly independent source of information.

The mach correction of static is done by computers using airspeed info, so at least the altitude information was "uncorrected", but probably much more exact than a lot of other information.

A Radalt can only work accurately if the aerials are pointing to the ground. Extreme bank and pitch angles will affect the reading, aerobatics render it useless.

The Radalts I've been used to only read to 2500 feet.

Laser laser l a s e r .... Just had to test this, why is the a in laser replaced sometimes with @?

Pitch and power is available in any jet for altitude and weight. This will maintain airspeed and approximate altitude. Pilots have to know how to fly to use this simple procedure. That is today's problem however.

PItch and Power is right. Bubbers said it all. My aircraft manual has a very nice page of pitch/power numbers just in case of missing radome after hail encounter or some other reason. And the radome area is where the pitot lives.

Talking about radar, wondering how many people remember Doppler Radar Nav which gave us ground speed/drift? I only knew about it because all of our planes of a certain age had the sticker: DOP INOP

RTO
Check out the l@ser pointer thread.
The automatic search engines that insert sponsored ads to support the site were picking up on the word l@ser and inserting ads for high powered l@ser pointers on a thread that was complaining about their use by idiots to target aircraft.

Anemometer?
 

Pitch and power are great tools, whether you've got a good IAS indication or not - belt and braces? But although the pitch values are easily remembered (remember the mnemonic "(DTD)585" for the B707?), the N1 or EPR settings are less so, unless you have noted the setting just before the IAS indications go AWOL. It can take a while to find the appropriate page in the QRH. On some a/c, the total fuel flow is a better tool than N1 or EPR, but it's not generally taught, and digital flowmeters don't seem to be as user-friendly as the old analogue ones.

I have a pipe dream in which the TAS would be detected by a kind of anemometer mounted at a suitable point on the fuselage. Provided the static pressure and TAT were available, the ADC could then calculate the CAS/IAS, and of course the Mach. But an anemometer might be even more prone to icing than a pitot head, and its RPM could be problematic...

Quote from glendalegoon:
Talking about radar, wondering how many people remember Doppler Radar Nav which gave us ground speed/drift? I only knew about it because all of our planes of a certain age had the sticker: DOP INOP

Suspect you may be betraying your age as sixty-plus? ;) Most Doppler-equipped a/c must have been retrofitted with INS by the late 1970s. IIRC, our B707s had their Doppler removed at the same time. As you no doubt remember, it wasn't much good over smooth sand or calm sea.

Mirkoni
The pitot system is cheap, simple and well understood. The AF447 system iced up due to the pitot heating system not being powerful enough to cope with the prevailing conditions at the time.

As usual, it was a combination of factors that lead to the aircraft crashing. The crew apparently chose to fly through a storm system rather than route around it which made the situation worse due to extreme turbulence while trying to recover the aircraft to stable flight. The sidestick controls didn't provide visual feedback to other cockpit members that the aircraft was being held stalled in a nose up attitude by the pilot flying. The crew were over reliant on automated systems and apparently unable to fly the aircraft manually without an accurate indication of airspeed.

A tragic accident where an otherwise perfectly serviceable aircraft crashed because the pilots were basically incapable of flying the aircraft manually - just one of an increasing number of such incidents.

Yeah, good old doppler. And my age is posted, heh heh.

Even better than the pitch/power rules-of-thumb is an inertially- based flight path marker with a HUD. Speed is still a problem if you are close to the mach limit. Less so if close to stall, as AoA indicators are really great for that. In fact, the AoA can be used at cruise speed, but it is very senstive, like fractions of a degree for a few knots.

BTW, my first inertial system was supplemented by a doppler ( nowadays they use GPS, but still use inertial vectors for accuracy in the tenths of a knot or even less). We could even do INS alignments inflight using the doppler and then providing a nav fix. This was late sixties/early seventies. You can do the same with the GPS.

Chris has a good idea about an old-fashioned anemometer, but the lazer doofer seems to offer a good backup for the hole-in-a-pipe things we use now.

There is a new technology called molecular tagging velocimetry. It works by tagging (triggering a chemical or quantum state change) in the medium (air) and then observing the relative movement of the marked molecules.

Molecular tagging velocimetry - Wikipedia, the free encyclopedia

The down side is that new technologies take time to gain regulatory acceptance.

Gouli
I fly an atmospheric research DC-8. It is normal when entering heavy precip for the airspeed indicators to go to zero, which is most noticeable in the ITCZ. The autopilot will not trip off and we all know about pitch/power. The INS/GPS continues to chug along with groundspeed, etc. We (science) even have a radio altimeter in the back that accurately reads much higher than the standard 2,500' reading on the pilot's panels.

Not even going out on a limb here, I have to say had AF447 been a DC-8 everyone would still be alive, provided the "pilots" didn't touch anything while the airspeeds read zero. Of course, the pilots wouldn't have had those "cool" little tables in front of them. Oh, and no autothrottles or magenta lines (except for the magenta lines on the pilot's iPads, of course).

Sorry. Had to say it. Them's the facts. Period.

Anemometer?
 

Could the anemometer icing heater work well enough?
Iced anemometers are likely to give a zero reading?

Maybe a spinning Sidewinder type wheel to measure power from the airflow, or a piezoelectric shear plate to measure stress on a surface not prone to icing.
Someone mentioned having an emergency pop-up spare at some point too.

I'm not sure any of these would work better in bad conditions than the current standard.