Why use only pitot-static system for altimeter/airspeed
This is what is so nice about this place. One comes along having heard a little bit about a subject and there is always someone around to fill one or two of the many gaps in ones rudimentary knowledge. Thank You Owain Glyndwr and Henry_crun for the explanations.
Join Date: Sep 2002
Location: La Belle Province
Posts: 2,179
Likes: 0
Received 0 Likes
on
0 Posts
There's still research going on into how to specify and address these kinds of conditions, as well, so the requirements are not frozen (ha ha) yet. (There's supposed also to be a flight test exercise out of Darwin, Australia, in the next couple of months to help with this; it might already have started)
Besides the taped over ports on one plane, then the "mud daubers" plugging up the pitot tube on another, the incident that gets the most attention is AF447. See the thousands of posts on that elsewhere.
I only had one air data freeze in 20 years, and was the static ports. Speed went crazy, but altitude on the steam gauge was stuck. BFD. Power, pitch and and so forth. 'course, I was cheating, as I had an inertial flight path marker and knew by heart the power and descent angle for the let down. Inertial groundspeed was a good crosscheck, even though it did not compensate for actual airspeed. But I also had a valid AoA indication if I got slow. Gotta use all the tools you have, ya think?
Good heaters on the probes/ports are essential. But seems we have had very few losses of the probes due to the heaters and then losses of the plane folowing losses of the probes.
The axiom to hold dear is: "Don't do something, just sit there". The AF447 accident is worth hours and hours of training lectures, and we'll never know what was going through the pilots' minds.
I only had one air data freeze in 20 years, and was the static ports. Speed went crazy, but altitude on the steam gauge was stuck. BFD. Power, pitch and and so forth. 'course, I was cheating, as I had an inertial flight path marker and knew by heart the power and descent angle for the let down. Inertial groundspeed was a good crosscheck, even though it did not compensate for actual airspeed. But I also had a valid AoA indication if I got slow. Gotta use all the tools you have, ya think?
Good heaters on the probes/ports are essential. But seems we have had very few losses of the probes due to the heaters and then losses of the plane folowing losses of the probes.
The axiom to hold dear is: "Don't do something, just sit there". The AF447 accident is worth hours and hours of training lectures, and we'll never know what was going through the pilots' minds.
Join Date: Jun 2011
Location: West of Offa's dyke
Age: 88
Posts: 476
Likes: 0
Received 0 Likes
on
0 Posts
@ Mad Flt Scientist
True, but the airworthiness authorities (FAA, EASA) had and have the power to make it retroactive if they think it necessary.
One should also remember, as pattern is full said, that:
With that model removed from the supply chain they presumably think retroactivity to be unnecessary.
But it is not (as of yet, anyway) retroactive, so older aircraft (which means virtually everyone) are potentially at-risk from a similar event
One should also remember, as pattern is full said, that:
Let's keep in mind that in the case of AF447, the problem was not the general technology of pitot systems, but a specific model from a specific manufacturer that proved to be less-than-perfect when it came to certain icing conditions.
While we're looking for a backup to the pitot-static systems, I could not resist a high tech backup that wasn't so high tech 40 years ago.
First, you must have a decent inertial system, GPS-aided or not.
Then you need a decent computer to provide movement across the earth - velocity and position.
If your nav computer notices that with your heading, that your course is not on the same heading, and also notices that your speed across the earth does not agree with the inertial/nav system numbers if you had zero "wind", then what's up? . Duh? We're flying in the "wind", the air mass. So a fairly simple computer uses actual velocity across the earth and heading with what it should be with no "wind"/air mass movement. You can now compute airspeed. Kinda the reverse of what we used to do way back with "forecast" winds and airspeed indicators and so forth to get course/groundspeed.
Back when the earth was still cooling, I flew a plane with that capability. It was very accurate, and ATC center controllers would frequently ask us what the winds were at our altitude/position. We didn't need actual indicated speed, but we could have easily reversed the equation. Mach could also be calculated using "system altitude" versus the pitot-static system altitude.
In short, if you have the modern systems and a few software folks, it is not that hard to provide "airspeed" thru the airmass without the pitot-static sensors AoA is a different matter.
First, you must have a decent inertial system, GPS-aided or not.
Then you need a decent computer to provide movement across the earth - velocity and position.
If your nav computer notices that with your heading, that your course is not on the same heading, and also notices that your speed across the earth does not agree with the inertial/nav system numbers if you had zero "wind", then what's up? . Duh? We're flying in the "wind", the air mass. So a fairly simple computer uses actual velocity across the earth and heading with what it should be with no "wind"/air mass movement. You can now compute airspeed. Kinda the reverse of what we used to do way back with "forecast" winds and airspeed indicators and so forth to get course/groundspeed.
Back when the earth was still cooling, I flew a plane with that capability. It was very accurate, and ATC center controllers would frequently ask us what the winds were at our altitude/position. We didn't need actual indicated speed, but we could have easily reversed the equation. Mach could also be calculated using "system altitude" versus the pitot-static system altitude.
In short, if you have the modern systems and a few software folks, it is not that hard to provide "airspeed" thru the airmass without the pitot-static sensors AoA is a different matter.
With that model removed from the supply chain they presumably think retroactivity to be unnecessary.
IIRC, engine inlet P2/T2 probes (which have occasionally had icing issues, especially the temp portion) get ~500 watts heat, while aircraft pitot probes get close to a kilowatt at high heat.
Of course, that doesn't help with the mud dauber or the taped over static. Sometimes there is no substitute for basic maintenance.
engine inlet P2/T2 probes (which have occasionally had icing issues, especially the temp portion) get ~500 watts heat, while aircraft pitot probes get close to a kilowatt at high heat.
Join Date: Jun 2011
Location: West of Offa's dyke
Age: 88
Posts: 476
Likes: 0
Received 0 Likes
on
0 Posts
@tdracer
I agree, but with one caveat - it shouldn't be hard provided that the design requirements are known. The problem has been, so far as I can see, that nobody has been able to describe the characteristics of the ice particles with enough certainty to define a foolproof requirement. Since 1996 the FAA have had a working group trying to do just that, interestingly their motivation was a series of engine power loss events not airspeed probes. A good summary of the situation can be found in AIAA 2006 - 206; The Ice Particle Threat to Engines in Flight.
Bingo! Making a pressure probe that won't ice isn't that hard - you can put as much heat into it as you want (and pretty much anywhere on the probe you want) without significantly affecting the measurement.
@ Skyjob, and thanks for your support, Okie...
The whole point is if your actual movement across the earth is not the same as it should be with zero airmass movement, then the delta is the wind. That's how our nav system calculated the wind values we would provide to the ATC guys. In those days, we would frequently see over 100 knots of wind at 20,000 - 25,000 feet during the winter in the Washington Center area and somewhat south. Our system also calculated true airspeed using the same algorithms, although we had an air data computer that was supposed to do the same thing. So on crossing the pond with a tanker, the tanker nav would ask us for TAS and ground speed and such. He would also ask for lat/long at a time hack. A minute later, we would notice a slight heading change and cracked up. The whole time we were looking at our steering "bug" in the HUD that was slightly off of our heading. The new heading centered the "bug", LOL. Was amazing that our system was so advanced to what the tankers had at the time.
If we knew the movement of the airmass, then it was easy to calculate our speed thru the airmass Equations upon request, but haven't done a lot of this since 1996 or so. Granted, the value we get would not be very accurate for a few seconds, but what the hell. The inertial is extremely accurate for milliseconds of velocity/flight path data. But for nav ya gotta smooth it out, and that takes a second ot two for all the Kalman filters to do their magic.
Mach is harder to calculate even if you know speed thru the airmass, and a basic inertial/GPS will provide altitude within 100 feet or so. Problem is temperature, and mach is directly related to the OAT ( outside air temp), just talk to a SR-71 pilot or nav.
Just thot I would add to all the other proposed widgets to back up the pitot tubes.
The whole point is if your actual movement across the earth is not the same as it should be with zero airmass movement, then the delta is the wind. That's how our nav system calculated the wind values we would provide to the ATC guys. In those days, we would frequently see over 100 knots of wind at 20,000 - 25,000 feet during the winter in the Washington Center area and somewhat south. Our system also calculated true airspeed using the same algorithms, although we had an air data computer that was supposed to do the same thing. So on crossing the pond with a tanker, the tanker nav would ask us for TAS and ground speed and such. He would also ask for lat/long at a time hack. A minute later, we would notice a slight heading change and cracked up. The whole time we were looking at our steering "bug" in the HUD that was slightly off of our heading. The new heading centered the "bug", LOL. Was amazing that our system was so advanced to what the tankers had at the time.
If we knew the movement of the airmass, then it was easy to calculate our speed thru the airmass Equations upon request, but haven't done a lot of this since 1996 or so. Granted, the value we get would not be very accurate for a few seconds, but what the hell. The inertial is extremely accurate for milliseconds of velocity/flight path data. But for nav ya gotta smooth it out, and that takes a second ot two for all the Kalman filters to do their magic.
Mach is harder to calculate even if you know speed thru the airmass, and a basic inertial/GPS will provide altitude within 100 feet or so. Problem is temperature, and mach is directly related to the OAT ( outside air temp), just talk to a SR-71 pilot or nav.
Just thot I would add to all the other proposed widgets to back up the pitot tubes.
Quote:
"Our system also calculated true airspeed using the same algorithms, although we had an air data computer that was supposed to do the same thing."
Errr, gums - I'm also confused. What sensor did your system derive its TAS from?
"Our system also calculated true airspeed using the same algorithms, although we had an air data computer that was supposed to do the same thing."
Errr, gums - I'm also confused. What sensor did your system derive its TAS from?
@ Chris
The TAS could have been the actual velocity thru the airmass, and we didn't have the computer use system altitude to calculate the CAS. That was just my own "invention" if we wanted to do it. The winds were easy, and we could display them on our nav panel. Our nav system was better than any other plane for another ten years or so. We had a super inertial, a doppler and a nav computer borrowed from the Apollo lunar landing module - an IBM PI 3, or close to it. So we had about 5 nav modes until it came down to the primitive methods we had all learned. We could even align the INS using the doppler, then get a nav fix for navigation and press on.
BTW, I flew from Hawaii to Guam in our "airmass mode" using predicted winds aloft and only my TAS from the air data computer and attitude/heading from the basic gauges. You know, the way we used to do back when the earth was still cooling and had no doppler or computers or.... I did it because I was in a flight alongside a tanker and the weather was supposed to be good. Besides, I had an ADF for homing if I got lost ( thinking of Amelia,,,,,)
I was about 15 miles off when getting to Guam. Not too shabby. Island was easy to spot due to the clouds above, so I would have made it without help.
The TAS could have been the actual velocity thru the airmass, and we didn't have the computer use system altitude to calculate the CAS. That was just my own "invention" if we wanted to do it. The winds were easy, and we could display them on our nav panel. Our nav system was better than any other plane for another ten years or so. We had a super inertial, a doppler and a nav computer borrowed from the Apollo lunar landing module - an IBM PI 3, or close to it. So we had about 5 nav modes until it came down to the primitive methods we had all learned. We could even align the INS using the doppler, then get a nav fix for navigation and press on.
BTW, I flew from Hawaii to Guam in our "airmass mode" using predicted winds aloft and only my TAS from the air data computer and attitude/heading from the basic gauges. You know, the way we used to do back when the earth was still cooling and had no doppler or computers or.... I did it because I was in a flight alongside a tanker and the weather was supposed to be good. Besides, I had an ADF for homing if I got lost ( thinking of Amelia,,,,,)
I was about 15 miles off when getting to Guam. Not too shabby. Island was easy to spot due to the clouds above, so I would have made it without help.
Join Date: Mar 2001
Location: I wouldn't know.
Posts: 4,497
Likes: 0
Received 0 Likes
on
0 Posts
@OK465
If i remember correctly it was removed in 2012 or early 2013, but i can't really recall the date.
Haven't been lucky enough to fly aircraft with the AoA gauge which seems to be indicated where some have the round dial RA display, there is an alternate purely digital RA display location on the bottom of the ADI part of the PFD. We have both RA displays in our fleet. The AoA display is available on the 737NG, 764, 777 and probably 787 and 748.
I did understand the same as you that it displays unfactored AoA, however with a red tick for stall AoA which is dependent on mach number. Boeing published a nice article about it in their aero magazine.
Wouldn't have helped in the majority of our unreliable airspeed cases as those were caused by frozen AoA vanes. Interestingly enough that resulted in wrong indications of airspeed, altitude, ground speed and wind info on the affected side. Luckily there was always only one sided affected and the ISFD doesn't take any AoA input and displayed correct information as well which made the crosscheck easier.
If i remember correctly it was removed in 2012 or early 2013, but i can't really recall the date.
Haven't been lucky enough to fly aircraft with the AoA gauge which seems to be indicated where some have the round dial RA display, there is an alternate purely digital RA display location on the bottom of the ADI part of the PFD. We have both RA displays in our fleet. The AoA display is available on the 737NG, 764, 777 and probably 787 and 748.
I did understand the same as you that it displays unfactored AoA, however with a red tick for stall AoA which is dependent on mach number. Boeing published a nice article about it in their aero magazine.
Wouldn't have helped in the majority of our unreliable airspeed cases as those were caused by frozen AoA vanes. Interestingly enough that resulted in wrong indications of airspeed, altitude, ground speed and wind info on the affected side. Luckily there was always only one sided affected and the ISFD doesn't take any AoA input and displayed correct information as well which made the crosscheck easier.
Join Date: Nov 2007
Location: dublin
Posts: 2
Likes: 0
Received 0 Likes
on
0 Posts
The GPS is a true life saver in a loss of indicated airspeed. When AF lost their airspeeds they only had to look at ground speed that existed at the time and then keep it there. It would have been around 440 since they had a bit of a headwind at the time I seem to remember but it doesn't matter. In still air it will be 480, and in a 200kt headwind it will be 280 but all you do is keep it there.
Your forecast wind on the flight plan is dead accurate these days and so all you need to do all the way to Paris is to subtract or add the headwind to find out what GPS groundspeed should be and fly that speed. You won' t be more than five its off your target. A lot better than what happened - being 350 kts too slow!
John
Your forecast wind on the flight plan is dead accurate these days and so all you need to do all the way to Paris is to subtract or add the headwind to find out what GPS groundspeed should be and fly that speed. You won' t be more than five its off your target. A lot better than what happened - being 350 kts too slow!
John
Join Date: Nov 2007
Location: dublin
Posts: 2
Likes: 0
Received 0 Likes
on
0 Posts
Hi there awblain
The GPS in AF 477 during the descent would have been reading less than 100 its. They were actually descending faster than that VERTICALLY! The pitot static system icing up can render the Altimeter and the airspeeds invalid - don' t know what the situation was with air france. The big clue is that the ground speed was 350 knots too slow. It simply amazes me that many airline pilots don't realise the value of GPS Ground Speed to stabilise the flight path Way back in the 80s, we flew the 737-200 with total failure of airspeed and altimeters back to a save landing using G/S and of course, pitch and power but the real saviour was ground speed. Say your landing speed is to be 130 and the headwind is 30 knots you just fly the approach at 100 kts ground speed and you will land at precisely 130 kts airspeed. The Tristar used to do this routinely and funnily enough, the Airbus does the same in managed speed mode (if it is working , and this is the problem).
It flies the approach using groundspeed to fine tune the approach speed.
Cheers for now
John
The GPS in AF 477 during the descent would have been reading less than 100 its. They were actually descending faster than that VERTICALLY! The pitot static system icing up can render the Altimeter and the airspeeds invalid - don' t know what the situation was with air france. The big clue is that the ground speed was 350 knots too slow. It simply amazes me that many airline pilots don't realise the value of GPS Ground Speed to stabilise the flight path Way back in the 80s, we flew the 737-200 with total failure of airspeed and altimeters back to a save landing using G/S and of course, pitch and power but the real saviour was ground speed. Say your landing speed is to be 130 and the headwind is 30 knots you just fly the approach at 100 kts ground speed and you will land at precisely 130 kts airspeed. The Tristar used to do this routinely and funnily enough, the Airbus does the same in managed speed mode (if it is working , and this is the problem).
It flies the approach using groundspeed to fine tune the approach speed.
Cheers for now
John
Join Date: Nov 2007
Location: dublin
Posts: 2
Likes: 0
Received 0 Likes
on
0 Posts
AF 477 stall
Another thing that most folks haven't spotted is this. Once they developed a deep stall with forward speed below 100 and vertical speed in excess of 100kts the angle of attack would have been greater than 45 degrees. That means that to unstall they would have had to lower the nose from what they had (15 deg nose up) to say 50 deg nose down to stand any chance of unstalling. That is a pitch change of 60 degrees - in the dark and with them in a state of panic not knowing what was going on. It was all over by the time they reached 25000 ft. Nothing after that was going to work. So, when you get into airspeed unreliable territory, fix onto ground speed and you will stay safe.
The rules are simple
Maintain pitch and power that exists and maintain the same ground speed. All is well.
The rules are simple
Maintain pitch and power that exists and maintain the same ground speed. All is well.
