Why use only pitot-static system for altimeter/airspeed
Quote from Desert185:
"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."
Presume you've not flown Airbus FBW? I have, and in previous years negotiated the ITCZ many times in jet types such as B707 and VC10 (both without A/THR), and the A310 and DC10 (both with A/THR). Must admit I never lost the ASIs, even on the rare occasions when the windshields were icing up. But GOULI should recognise that there isn't always a clear way through the ITF, and that the turbulence encountered by AF447 was far from "extreme".
Haven't flown one, but the A330 (even the shorter 200) probably handles fairly similarly to the DC8 in chop. It's a big, stable aircraft. Admittedly, the AP was lost because of the UAS. (The A/THR wouldn't be usable for the pitch/power technique anyway.) The AF447 PF had the advantage of Pitch Alternate law, which would make pitch control easy, but he had to keep the wings level himself. Why he chose consistently to pull up will always remain open to conjecture - I've floated ideas myself. Like you say on your a/c, the trick is to maintain the status-quo.
Now with plenty of time to read and think, I think there is a growing issue in aviation of loss of basic flying skills; partly due to automation and complacency. Airbus has to take its share of the blame, including that for making the MTBF so much longer than it was even in my youth... But so do the airlines and the regulators.
Hats off to anyone who's involved in airborne research into a/c behaviour in adverse wx, particularly in a veteran airframe!
"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."
Presume you've not flown Airbus FBW? I have, and in previous years negotiated the ITCZ many times in jet types such as B707 and VC10 (both without A/THR), and the A310 and DC10 (both with A/THR). Must admit I never lost the ASIs, even on the rare occasions when the windshields were icing up. But GOULI should recognise that there isn't always a clear way through the ITF, and that the turbulence encountered by AF447 was far from "extreme".
Haven't flown one, but the A330 (even the shorter 200) probably handles fairly similarly to the DC8 in chop. It's a big, stable aircraft. Admittedly, the AP was lost because of the UAS. (The A/THR wouldn't be usable for the pitch/power technique anyway.) The AF447 PF had the advantage of Pitch Alternate law, which would make pitch control easy, but he had to keep the wings level himself. Why he chose consistently to pull up will always remain open to conjecture - I've floated ideas myself. Like you say on your a/c, the trick is to maintain the status-quo.
Now with plenty of time to read and think, I think there is a growing issue in aviation of loss of basic flying skills; partly due to automation and complacency. Airbus has to take its share of the blame, including that for making the MTBF so much longer than it was even in my youth... But so do the airlines and the regulators.
Hats off to anyone who's involved in airborne research into a/c behaviour in adverse wx, particularly in a veteran airframe!
Nice, Desert, and Okie was a frequent contributor on the 447 threads.
As one said on the thread, "don't just do something, just sit there".
The 'bus reversion modes do not seem to go to "attitude" hold when the air data is deemed unreliable . They just go back to "manual", such as it is in the FBW system they have. And that isn't too bad, considering the system tries to achieve 1 gee adjusted for pitch attitude. So it resembles an "attitude" control mode. But the pilots tried to do "something" versus taking a moment to analyze what they had and what the jet was doing. Still makes me cry.
Only had one pitot-static freeze and it was the static pressure. So IAS went off the charts as I descended. Fortunately, I had the super flight path marker in my HUD and knew the power settings for the descent. Below 5,000 feet the radar altimiter kicked in and finally the sensors thawed out. 'course, I had good AoA for the approach if the air data stuff remained frozen. This was back in early 70's
I cannot emphasize the value of an inertial flight path marker, and a HUD makes it even more useful. Airspeed goes tango uniform, BFD. You can see your flight path and stay where you want it - TOGA, cruise, descent on a precision approach or non-precision approach ( think Huntsville and Asiana). Sheesh. It was like cheating. With a flight director display, you don't even need the HUD. Just line up the symbols.
Inertial systems these days are very cheap compared to when I first saw them. One neat thing was the inertial vertical velocity bug next to the altitude scale on the HUD. Beat the hell outta the baro indicator, and I think about Asiana. The suckers are also independent of the baro data for short term use, but keeping the FPM on the horizon line was peachy keen when baro things went tango uniform. Then was the power settings we all knew from experience and the manuals.
As one said on the thread, "don't just do something, just sit there".
The 'bus reversion modes do not seem to go to "attitude" hold when the air data is deemed unreliable . They just go back to "manual", such as it is in the FBW system they have. And that isn't too bad, considering the system tries to achieve 1 gee adjusted for pitch attitude. So it resembles an "attitude" control mode. But the pilots tried to do "something" versus taking a moment to analyze what they had and what the jet was doing. Still makes me cry.
Only had one pitot-static freeze and it was the static pressure. So IAS went off the charts as I descended. Fortunately, I had the super flight path marker in my HUD and knew the power settings for the descent. Below 5,000 feet the radar altimiter kicked in and finally the sensors thawed out. 'course, I had good AoA for the approach if the air data stuff remained frozen. This was back in early 70's
I cannot emphasize the value of an inertial flight path marker, and a HUD makes it even more useful. Airspeed goes tango uniform, BFD. You can see your flight path and stay where you want it - TOGA, cruise, descent on a precision approach or non-precision approach ( think Huntsville and Asiana). Sheesh. It was like cheating. With a flight director display, you don't even need the HUD. Just line up the symbols.
Inertial systems these days are very cheap compared to when I first saw them. One neat thing was the inertial vertical velocity bug next to the altitude scale on the HUD. Beat the hell outta the baro indicator, and I think about Asiana. The suckers are also independent of the baro data for short term use, but keeping the FPM on the horizon line was peachy keen when baro things went tango uniform. Then was the power settings we all knew from experience and the manuals.
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Unusual airspeed sensor
One little known airspeed sensor used ion flow to measure airspeed. As I recollect it measured electrical ion flow transverse to airflow and a combination of devices could sense airflow in any direction.
It was invented by Ed Durbin, head of Kaiser Electronics, and I had the task of interfacing it with the AH-1T electronics. That aircraft could and probably did interface with anything and everything in the inventory and then some.
As I recollect the sensor was only developed at helicopter speeds and never saw actual airframe service. Dunno if it could be extended to higher speeds. Very useful to have the full vector, rather than just forward speed.
PDF download:
http://www.aerovelco.com/papers/Durb...ctorSensor.pdf
Patent:
http://www.docstoc.com/docs/47436556...Patent-4680962
One little known airspeed sensor used ion flow to measure airspeed. As I recollect it measured electrical ion flow transverse to airflow and a combination of devices could sense airflow in any direction.
It was invented by Ed Durbin, head of Kaiser Electronics, and I had the task of interfacing it with the AH-1T electronics. That aircraft could and probably did interface with anything and everything in the inventory and then some.
As I recollect the sensor was only developed at helicopter speeds and never saw actual airframe service. Dunno if it could be extended to higher speeds. Very useful to have the full vector, rather than just forward speed.
PDF download:
http://www.aerovelco.com/papers/Durb...ctorSensor.pdf
Patent:
http://www.docstoc.com/docs/47436556...Patent-4680962
Agreed, OK465, the FPV is great SA device. In the Global, we have it, too. Coasting in on northern South America sometime back around 1am local, we plunged into an ITCZ storm that didn't show on radar or lightning detection--severe turbulence, lost about 1,500'. The "moose antlers" we're the clue to start down, now! Shaker fired for just a second, but I'd already started unloading. The FPV showed exactly how much pitch down it would take to stay out of the stall. Anyway, popped out into the clear.
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OK465, you might reread your FCOM, the NG FPV is not completely inertial, it does receive baro input and is dependent on the ADC. Simulators might display it differently though as the FPV is not a critical system for operation its simulation often is quite sketchy.
Text from the manual regarding the FPV:
If one is absolutely sure that only the speed indication has failed but the barometric input is still correct he can use the FPV in those situations, if only the slightest doubt is there that barometric input might be impaired as well, it is better not to use it.
Aside from that it is of course a nice tool and can be of use in real world flying as well, even if of dubious note in non normal situations.
Text from the manual regarding the FPV:
The FPV shows the Flight Path Angle (FPA) above or below the horizon line and drift angle left or right of the pitch scale's center. The FPA uses inertial and barometric altitude inputs. The vertical FPA is unreliable with unreliable primary altitude displays.
Aside from that it is of course a nice tool and can be of use in real world flying as well, even if of dubious note in non normal situations.
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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.
A backup airspeed and Mach could simply be calculated in the airplane's computer system from altitude, AoA, weight and loadfactor.
LDA is a truly independent system, you may possibly even use it through the cockpit windows, so it is well protected from the environment and for example any ice blocking the light path is evidient to the crew.
On the other hand, why do we need a new system, if all it takes are old pilots...
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LDA is a truly independent system, ...
Could control effectiveness be used to calculate airspeed? For example, perhaps the FBW system could induce a small, brief, rudder deflection and measure the resulting yaw rate?
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Due to the inertia on large(r) aircraft this yaw rate would not be noticeable, so I suspect this is not a good option and no real alternative.
As many have pointed out the pitot static system has its limitations but it is relatively well understood. Problems can arise in parts of the system and cause erroneous information but this can be overcome by flying technique.
The problem remains how to be made aware of such failures.
This is an area that needs attention and development, as a recently released image of a heated pitot tube shows, where the tip was actually not heated due to design, where Boeing has since advised of a retrofit by switching polarity of the connectors.
Rather then looking for alternatives, efforts should be made to increase feedback to pilots when a system may have failed, as this is not always annunciated until a feedback loop records a complete failure. Such information would be useful to crew so to know when to ignore the affected instruments and use flying techniques to overcome a downgraded system indication.
We have all been told to fly the magenta line and to trust our instruments at some point in training, as well as to ignore those flight directors and instruments under certain conditions.
It is imperative that recognition of these conditions occurs. It may not always be easy when flying highly automated aircraft to notice such conditions as a crew. A feedback loop that would indicate a problem before failure would enhance the awareness of the crew to an impending failure or equipment downgrade.
As many have pointed out the pitot static system has its limitations but it is relatively well understood. Problems can arise in parts of the system and cause erroneous information but this can be overcome by flying technique.
The problem remains how to be made aware of such failures.
This is an area that needs attention and development, as a recently released image of a heated pitot tube shows, where the tip was actually not heated due to design, where Boeing has since advised of a retrofit by switching polarity of the connectors.
Rather then looking for alternatives, efforts should be made to increase feedback to pilots when a system may have failed, as this is not always annunciated until a feedback loop records a complete failure. Such information would be useful to crew so to know when to ignore the affected instruments and use flying techniques to overcome a downgraded system indication.
We have all been told to fly the magenta line and to trust our instruments at some point in training, as well as to ignore those flight directors and instruments under certain conditions.
It is imperative that recognition of these conditions occurs. It may not always be easy when flying highly automated aircraft to notice such conditions as a crew. A feedback loop that would indicate a problem before failure would enhance the awareness of the crew to an impending failure or equipment downgrade.
I have to go with Okie on the utility of a flight path vector, not a flight path angle.
The two jets I flew that used the inertial system to display our flight path vector showed actual vector in space, not simply angle above/below the horizon. 'course, this was because we used the system for weapon delivery, so "drift" was automatically compensated for, as well as other parameters. No more TLBAR when bombing ( that looks 'bout right, heh heh).
The basic inertial platform was also used for the "attitude" indicator ( steam gauge). If the accelerometers went TU, then the sucker still worked like any other attitude indicator, and could be re-initialized as with the older systems, just no velocity vector. remember the J-8 from the 60's?
The beauty of our systems was their independence of air data. As long as we had electrons, we had an excellent display to use if the air data was completely gone, and our vertical position in space was excellent. Now, it is true that we used air data to smooth the vertical position in space ( system altitude), but if the air data went TU, no big deal. Inertial drift of the 70's and 80's platforms was less than one knot/hour in all reference axis.
To summarize, using an inertial platform to provide flight path data and speed and altitude is very useful if the pitot-static system goes west. Granted, the plane flies in the air mass, so actual speed thru the airmass is important. But for short periods, it's a life-saver until your air data sensors thaw out. Even if they don't, you can use the rules-of-thumb for power and attitude. Oh yeah, we had AoA sensors that might still be working if the other sensors froze up. Fine for stall avoidance, not so useful for limiting mach parameter.
The two jets I flew that used the inertial system to display our flight path vector showed actual vector in space, not simply angle above/below the horizon. 'course, this was because we used the system for weapon delivery, so "drift" was automatically compensated for, as well as other parameters. No more TLBAR when bombing ( that looks 'bout right, heh heh).
The basic inertial platform was also used for the "attitude" indicator ( steam gauge). If the accelerometers went TU, then the sucker still worked like any other attitude indicator, and could be re-initialized as with the older systems, just no velocity vector. remember the J-8 from the 60's?
The beauty of our systems was their independence of air data. As long as we had electrons, we had an excellent display to use if the air data was completely gone, and our vertical position in space was excellent. Now, it is true that we used air data to smooth the vertical position in space ( system altitude), but if the air data went TU, no big deal. Inertial drift of the 70's and 80's platforms was less than one knot/hour in all reference axis.
To summarize, using an inertial platform to provide flight path data and speed and altitude is very useful if the pitot-static system goes west. Granted, the plane flies in the air mass, so actual speed thru the airmass is important. But for short periods, it's a life-saver until your air data sensors thaw out. Even if they don't, you can use the rules-of-thumb for power and attitude. Oh yeah, we had AoA sensors that might still be working if the other sensors froze up. Fine for stall avoidance, not so useful for limiting mach parameter.
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The NG FPV has a purely inertial mode and more importantly can still be displayed even if both ADRs are 'TU'. It can function and be useful with only the IRUs to drive it. Naturally whether you use it or not is up to you. Follow your FCOM of course.
If you mean with "AoA" information the pitch limit indication which is displayed with flaps deployed or activated stick shaker, those are not pure AoA driven either, they do take inputs from the AoA vane (no surprise there), ADIRU outputs (both IR and AD part), anti-ice controls, wing configuration, thrust and FMC outputs. Quite a lot of information mixed together and computed by the SMYD computers.
I would be very cautious about simulator trials for that stuff, many level D simulators are not very good at correctly simulating some of the things that are not used with standard boeing SOPs, like for example the edge of envelope behavior of the FPV.
I am curious about a flight path marker display/system/implentation such as Denti describes. I can understand a crude system using AoA and actual pitch attitude, but why? Inertial platforms these days are extremely reliable and accurate and cheap compared to the stuff we had in the 70's and 80's.
The inertial flight path vector we had was no sierra inertial with respect to Mother Earth. No AoA inputs, no baro inputs. I mean, why? Sheesh. The inertial patform was also used to present basic attitude reference accurate to maybe 0.01 of a degree. Add the accelerometers and get the flight path vector, maybe to 0.02 of a degree ( figure 1 milliradian or so). Add the HUD pitch lines from the basic platform and it was a simple matter to see if you were climbing or descending. Made night takeoffs and missed approaches very comfortable when cross-checking the steam gauges.
Guess I better look up all the avionics on these "new" jets, ya think?
The inertial flight path vector we had was no sierra inertial with respect to Mother Earth. No AoA inputs, no baro inputs. I mean, why? Sheesh. The inertial patform was also used to present basic attitude reference accurate to maybe 0.01 of a degree. Add the accelerometers and get the flight path vector, maybe to 0.02 of a degree ( figure 1 milliradian or so). Add the HUD pitch lines from the basic platform and it was a simple matter to see if you were climbing or descending. Made night takeoffs and missed approaches very comfortable when cross-checking the steam gauges.
Guess I better look up all the avionics on these "new" jets, ya think?
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Chu Chu :
Skyjob:
Reminded me of an idea I had a while back:
I believe some AOA sensors have an integral test driver (think analog meter movement coil) to move the vane, in normal use this is part of an end to end test of the system.
My idea was to use the test driver to force a small defiection (up and down) of the AoA vane, the drive current required for the deflection should be proprotional to airspeed. (actually density and speed?)
Way beyond my knowledge to specifiy other parameters that would be needed to calculate air speed but should be possible.
One nice thing about this is that it would require no additional HW, probably would need a enhanced AoA sensor (calibrated drive) but that would be a direct replacement.
Just a thought, will wait patiently for the royalty checks :-)
Could control effectiveness be used to calculate airspeed? For example, perhaps the FBW system could induce a small, brief, rudder deflection and measure the resulting yaw rate?
Due to the inertia on large(r) aircraft this yaw rate would not be noticeable, so I suspect this is not a good option and no real alternative.
I believe some AOA sensors have an integral test driver (think analog meter movement coil) to move the vane, in normal use this is part of an end to end test of the system.
My idea was to use the test driver to force a small defiection (up and down) of the AoA vane, the drive current required for the deflection should be proprotional to airspeed. (actually density and speed?)
Way beyond my knowledge to specifiy other parameters that would be needed to calculate air speed but should be possible.
One nice thing about this is that it would require no additional HW, probably would need a enhanced AoA sensor (calibrated drive) but that would be a direct replacement.
Just a thought, will wait patiently for the royalty checks :-)
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Some good posts and discussion in this thread.
I don't think anyone's mentioned this but this discussion reminds me of a (hang on...Google it now)...B757 Aeroperú Flight 603 crash back in 1996.
The crew crashed a perfectly good airplane because they allowed a failed static port (taped up during a wash the night before and missed during preflight) to confuse them enough to lose control.
Basically this crash was caused by the crews inability to recognize a simple failure and work around it.
Back to the OP topic....why do we rely on such old technology instead of some new digital technology? Because when it works it works really well and usually it works...it's a very reliable technology.
So...what about when it doesn't work? This crew failed to recognize the problem and work around it....Probably confused because there were multiple failures caused by more than one piece of tape. there are so many other ways to verify altitude and airspeed already on board....the GPS systems being just one of them.
The pressurization controller may also have been taped up but they could have slowly depressurized the airplane in an extreme case to verify altitude. An airplane can never have a significant negative differential so as long as the cabin altitude is say...ten thousand feet...the airplane is at least ten thousand feet or higher and that reading is from inside the cabin...it wouldn't matter if every port on the outside of the plane was taped up.
But...still....what was said over and over in this thread...pitch and power...a B757 will fly just like a single engine Piper or Cessna.
Oh yes....do a thorough pre-flight walk around too.
I don't think anyone's mentioned this but this discussion reminds me of a (hang on...Google it now)...B757 Aeroperú Flight 603 crash back in 1996.
The crew crashed a perfectly good airplane because they allowed a failed static port (taped up during a wash the night before and missed during preflight) to confuse them enough to lose control.
Basically this crash was caused by the crews inability to recognize a simple failure and work around it.
Back to the OP topic....why do we rely on such old technology instead of some new digital technology? Because when it works it works really well and usually it works...it's a very reliable technology.
So...what about when it doesn't work? This crew failed to recognize the problem and work around it....Probably confused because there were multiple failures caused by more than one piece of tape. there are so many other ways to verify altitude and airspeed already on board....the GPS systems being just one of them.
The pressurization controller may also have been taped up but they could have slowly depressurized the airplane in an extreme case to verify altitude. An airplane can never have a significant negative differential so as long as the cabin altitude is say...ten thousand feet...the airplane is at least ten thousand feet or higher and that reading is from inside the cabin...it wouldn't matter if every port on the outside of the plane was taped up.
But...still....what was said over and over in this thread...pitch and power...a B757 will fly just like a single engine Piper or Cessna.
Oh yes....do a thorough pre-flight walk around too.
Lifeafteraviation
Aeroperu had many things going against it - aside from no usable altitude or airspeed and near zero visibility, they were being mislead by ATC telling them their altitude, with no one realizing that the altitude that ATC saw was just as corrupted as what they saw on their displays. In 20-20 hindsight, they should have used the radio altimeter, but that would have taken some out of the box thinking since they believed that, at the time they hit the water, they were well above the altitude where the RA was meaningful. In short, I doubt many pilots - even old school stick and rudder types - could have made a happy outcome from the pile of
they were given.
BTW, in the aftermath of Aeroperu and Bergenair accidents (which coincidentally happened at about the same time, both 757s), Boeing developed specific AFM procedures for dealing with bad or suspect air data indications.
I don't know if those procedures ever had to come into play...
Aeroperu had many things going against it - aside from no usable altitude or airspeed and near zero visibility, they were being mislead by ATC telling them their altitude, with no one realizing that the altitude that ATC saw was just as corrupted as what they saw on their displays. In 20-20 hindsight, they should have used the radio altimeter, but that would have taken some out of the box thinking since they believed that, at the time they hit the water, they were well above the altitude where the RA was meaningful. In short, I doubt many pilots - even old school stick and rudder types - could have made a happy outcome from the pile of
they were given.BTW, in the aftermath of Aeroperu and Bergenair accidents (which coincidentally happened at about the same time, both 757s), Boeing developed specific AFM procedures for dealing with bad or suspect air data indications.
I don't know if those procedures ever had to come into play...
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...
What about a 'tongue' made of suitable material sticking out perpendicular to the aircraft fuselage which has no moving parts and which is bent back by the airflow? There would be no pipe to get blocked by insects or ice, and one would just measure the amount of bend electrically caused by the airflow to compute airspeed. You would heat the tongue to keep it de-iced, but this arrangement seems much more robust to me.
Any instrumentation designers or engineers care to comment?
Uplinker- that's exactly what the likes or early Dehaviland Moths were equipped with!!
Galleries | Dehavilland Gypsy Moth Air Speed Indicator - 1929 | Flickr - Photo Sharing!
Galleries | Dehavilland Gypsy Moth Air Speed Indicator - 1929 | Flickr - Photo Sharing!
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Okaaaay - let's invent a new one then.
Use three small transducers mounted in a straight line fore-and-aft. Emit pulses of sound from the centre transducer and receive them on the forward and aft ones. Measure the time difference. Compute speed.
Of course noise will be a nuisance, but by using spread spectrum sound and suitable processing this would not be a problem.
Would it attract any attention? Nope, not here. Nobody understood the ion anemometer I posted on page two and I guess nobody will understand this one, apart from Gums who will recognise it as a use of doppler.
Retires all bitter and twisted to kitchen/workshop. Hat, coat, soldering iron....
Use three small transducers mounted in a straight line fore-and-aft. Emit pulses of sound from the centre transducer and receive them on the forward and aft ones. Measure the time difference. Compute speed.
Of course noise will be a nuisance, but by using spread spectrum sound and suitable processing this would not be a problem.
Would it attract any attention? Nope, not here. Nobody understood the ion anemometer I posted on page two and I guess nobody will understand this one, apart from Gums who will recognise it as a use of doppler.
Retires all bitter and twisted to kitchen/workshop. Hat, coat, soldering iron....