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Thread: Pitot Tube Failure

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    Default Pitot Tube Failure

    Hello everyone,,
    Involved in a discussion of the computerization of recently produced automobiles, trying to make an analogy to fly by wire. Attempting to reference pitot tube failure. Got hung up on how fly by wire systems compensate for loss of pitot tube sensing. I know that not all systems are dependent on working pitot tubes. But all I remember of the long detailed discussions of crashes involving pitot tubes is something about power and attitude. So maybe someone can remind me of where the pilots turn when they no longer have air speed data. It is my impression that air speed is very critical to flight management, but pilots can still aviate after the air speed is unreliable.

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    Quote Originally Posted by Jhonmicky View Post
    Got hung up on how fly by wire systems compensate for loss of pitot tube sensing.
    They don't, unless you consider losing protections, degrading the flight law and the autopilot stopping working a "compensation".

    But all I remember of the long detailed discussions of crashes involving pitot tubes is something about power and attitude. So maybe someone can remind me of where the pilots turn when they no longer have air speed data.
    Exactly. Pilots (not fight automation or fly-by-wire systems) turn to P+P=P, which means "Power + Pitch = Performance", which in turn means what combination of power (thrust) setting and pitch attitude will give you the desired airspeed and vertical speed.

    A good question could be why, to this day and to my knowledge, P+P=P has not been incorporated in the flight automation logic. It doesn't seem to be so complicated.

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    Quote Originally Posted by Jhonmicky View Post
    Hello everyone,,
    Involved in a discussion of the computerization of recently produced automobiles, trying to make an analogy to fly by wire. Attempting to reference pitot tube failure. Got hung up on how fly by wire systems compensate for loss of pitot tube sensing. I know that not all systems are dependent on working pitot tubes. But all I remember of the long detailed discussions of crashes involving pitot tubes is something about power and attitude. So maybe someone can remind me of where the pilots turn when they no longer have air speed data. It is my impression that air speed is very critical to flight management, but pilots can still aviate after the air speed is unreliable.
    Redundancy and reversion. FBW needs agreement from two pitot tube sensors to function in Normal Law (with flight envelope protections). Therefore there are at least three pitots. If one pitot is lost, there are still at least two in agreement, so the condition is considered fail-operational, the FBW remains in Normal Law. However, if multiple pitots are lost, and there is no agreement between at least two of them, FBW degrades to a reversion control law where certain protections are no longer in force. FBW continues to function however.

    Some aircraft have a red-green speed range scale that replaces the normal speed-tape on the primary flight display in the event of a loss of pitot airspeed data. The scale is driven by calculations using angle-of-attack data instead of pitot data, and allows the pilots to fly within a safe (green) speed range until the pitot data restores (multiple pitot failure is almost always the result of ice ingestion that overwhelms the pitot heater's ability to clear it, but typically only lasts a short time before the heater can restore function.) Otherwise, pilots must use the P + P = P formula Gabriel mentioned. Values for pitch and power are provided in the Quick Reference Guide that pilots keep close at hand for emergencies.

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    Quote Originally Posted by Gabriel View Post
    A good question could be why, to this day and to my knowledge, P+P=P has not been incorporated in the flight automation logic. It doesn't seem to be so complicated.
    Maybe because that's not a super accurate way to measure speed: it takes a while for a plane to actually settle on an airspeed. (Ever follow any of the Garrison stuff about getting the airplane "on the step" like you do a speed boat?...it's kind of a fallacy, but then again, there's some fundamental truths, too).

    Don't get me wrong, it's much preferred to a relentless pull up.

    And, we have a new insight into Evan...so if the system goes crazy and you don't have airspeed, you cannot choose a known and familiar power and pitch from memory, but you CAN look it up in an acronym book. I guess that eliminates you from accidentally using a combination from your 150 days. Fascinating.
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    Quote Originally Posted by 3WE View Post
    And, we have a new insight into Evan...so if the system goes crazy and you don't have airspeed, you cannot choose a known and familiar power and pitch from memory, but you CAN look it up in an acronym book. I guess that eliminates you from accidentally using a combination from your 150 days. Fascinating.
    First the known (memorized) and familiar settings to stabilize, then the more weight/altitude specific ones from the QRH. It's not a new insight 3WE, and it's not that fascinating.

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    Quote Originally Posted by 3WE View Post
    Maybe because that's not a super accurate way to measure speed: it takes a while for a plane to actually settle on an airspeed.
    I won't argue that. But will you argue that a flight control computer + autopilot can do it better, more accurately, and more reliably than a human pilot startled by he sudden disconnection of the autopilot out of nowhere?

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    Quote Originally Posted by Evan View Post
    Some aircraft have a red-green speed range scale that replaces the normal speed-tape on the primary flight display in the event of a loss of pitot airspeed data. The scale is driven by calculations using angle-of-attack data instead of pitot data, and allows the pilots to fly within a safe (green) speed range until the pitot data restores.
    This system was optional in the planes types that even offered it as an option.
    And then I heard critiques because AoA is very non-sensitive to airspeed at high speeds, meaning that you need big variations in airspeed to make a meaningful change in AoA (always keeping 1G) and that, with that considered, it was even less accurate than P+P=P (although I don't understand why, since pitch -one of the P- is as sensitive as AoA, in delta-pitch = delta-AoA if flight path remains constant).
    But anyway, I've seen this system being dis-recommended by some experts.

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    Quote Originally Posted by Gabriel View Post
    ***will you argue that a flight control computer + autopilot can do it better, more accurately, and more reliably than a human pilot startled by he sudden disconnection of the autopilot out of nowhere?***
    That wasn't what you asked earlier.

    The answer to your question is very simple: "No- computers are generally a lot faster and more accurate and therefore, generally do a better job"

    The answer to the real question is not so simple: "Something has gone awry and we don't know what the airspeed is- do I invoke a second system (which may or may not be functioning correctly) and which will have +/- 10 knots accuracy as the plane settles, OR do I turn the system over to the human who often has a little bit better perception of what's actually going on.

    I would further argue that the loss of an airspeed indication usually comes from a rather minor problem, so maybe it's generally better to dump it in the pilots lap so he can look up pitch and power values in Evan's acronym book since he can't revert to familiar settings.

    Are there some exceptions...certain persons in stormy weather in the dark of night, overload, panic...yes?.

    I guess we need more lines of code, but be sure to not let a DCAS type flaw sneak in.
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    Quote Originally Posted by Gabriel View Post
    Blah blah blah- I heard critiques blah blah- this system being dis-recommended by some experts.
    Because???

    Blah blah blah Translation: [It's not all that accurate.]

    Also- watch a plane land in brisk, but-not-extreme wind...lot's of pretty remarkable pitch changes...Guess what the airspeed and glideslope and lateral alignment looks like: Amazingly dang almost dead on AND a hell of a lot better than you or I could ever do.
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    Quote Originally Posted by 3WE View Post
    The answer to the real question is not so simple: "Something has god awry and we don't know what the airspeed is- do I invoke a second system (which may or may not be functioning correctly) and which will have +/- 10 knots accuracy as the plane settles, OR do I turn the system over to the human who often has a little bit better perception of what's actually going on.
    You do nothing. The autoflight reacts to the loss of airspeed data by entering a flashing amber UAS mode, essentially a speed hold on the autothrottle and a pitch hold on the vertical guidance, using values exactly right for the current weight and altitude to maintain the previous airspeed at the previous level altitude (more or less, but within RVSM tolerance). The system also monitors AoA to ensure that the plane remains safely in the envelope while FADEC or EEC assures that power setting is resulting in expected thrust. The system knows attitude, AoA, static air data, weight, configuration and thrust, thus it can fly a safe airspeed derived from this data. The pilots are alerted to the mode with an aural alert and may take over manual control or opt to work the problem while autoflight does the flying. Guess which they would choose...

    No upset = no pilot error.

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    Quote Originally Posted by Evan View Post
    You do nothing.
    Evan, stop the Pavlovian regurgitation of acronym manuals and read the words.

    Gabieee asked a very specific question as to why P+P=P isn't used as a computerized speed calculation method. He also discussed autopilot disconnects.

    Based on ~75 personal experiences of watching P+P take a few minutes to settle on an airspeed, I offered the discussion that MABYE because it wasn't super accurate over the short term. What is your experience on using P+P=P to arrive at a target airspeed?

    I love the concept of P+P=P being a powerful way to keep between stall and overspeed...I concede, it's not perfect for nailing a target speed. If there's a problem on the plane, follow procedures, but it's OK if you happen to know some fundamental stuff that's behind the procedures.

    Gabieee agreed.

    This is fundamental aeronautical stuff...I know you love to have it buried in cryptic acronyms in a computer, but the USA and forum polices allow discussion of that which you disdain.
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    Quote Originally Posted by 3WE View Post
    Evan, stop the Pavlovian regurgitation of acronym manuals and read the words.

    Gabieee asked a very specific question as to why P+P=P isn't used as a computerized speed calculation method. He also discussed autopilot disconnects.

    Based on ~75 personal experiences of watching P+P take a few minutes to settle on an airspeed, I offered the discussion that MABYE because it wasn't super accurate over the short term. What is your experience on using P+P=P to arrive at a target airspeed?

    I love the concept of P+P=P being a powerful way to keep between stall and overspeed...I concede, it's not perfect for nailing a target speed. If there's a problem on the plane, follow procedures, but it's OK if you happen to know some fundamental stuff that's behind the procedures.

    Gabieee agreed.

    This is fundamental aeronautical stuff...I know you love to have it buried in cryptic acronyms in a computer, but the USA and forum polices allow discussion of that which you disdain.
    I think you are playing devil's advocate. An automated flight version of P + P is going to arrive at the target airspeed faster than a human pilot and it's going to target that airspeed better and it's going to remain there more reliably. Now consider the scenario: You are 98% N1 and 0° pitch when the airspeeds are lost. The system instantly reorients the aircraft to 99% N1 and 2.5° pitch. How long is it going to take for the speed change to settle?

    I have long agreed with Gabriel on this. I don't see a good reason not to have an autopilot reversion mode for UAS.

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    Quote Originally Posted by Evan View Post
    You do nothing. The autoflight reacts to the loss of airspeed data by entering a flashing amber UAS mode, essentially a speed hold on the autothrottle and a pitch hold on the vertical guidance, using values exactly right for the current weight and altitude to maintain the previous airspeed at the previous level altitude (more or less, but within RVSM tolerance). The system also monitors AoA to ensure that the plane remains safely in the envelope while FADEC or EEC assures that power setting is resulting in expected thrust. The system knows attitude, AoA, static air data, weight, configuration and thrust, thus it can fly a safe airspeed derived from this data. The pilots are alerted to the mode with an aural alert and may take over manual control or opt to work the problem while autoflight does the flying. Guess which they would choose...

    No upset = no pilot error.
    3we will not like it, but I 100% agree with the above. Even more, to ensure you stay within RVSM, the pitch mode can be actually an altitude control mode and the thrust can be adjusted so as the plane flies the selected altitude with the target pitch. It would be very difficult (I would venture impossible) to control altitude within RVSM compliant margins if you try to control altitude with an open loop or even with a closed loop that relies on the throttles, because engine and speed react more slowly than pitch and vertical speed. With this alternative that I am proposing, you really don't care too much if the pitch wanders 0.3 degrees above and below the target while the throttle adjusts up and down trying to find the right speed that will keep the altitude nailed, while the elevator takes care of assuring that the altitude remains nailed.

    Even more, this would be easily scalable to use the autopilot and autothrottle in a wide range of speeds and vertical speeds, which would allow not just to keep the current speed and altitude but also to change it in case the issue doesn't clear soon or in case you need to climb or descent to clear or get out of whether. Just let the system pick the right pitch goal for the selected speed and add or subtract from that speed the flight path angle that would result from te selected vertical speed combined with the speed. And even more, the manual flight can also take advantage of this by G-on stick-or pitch-on-stick, but always with autothrottles in selected speed, and the pitch can be limited with a good margin up and down from the points where the throttles would not be able to maintain the selected speed at idle (lower pitch limit) or at climb thrust (upper pitch limit).

    And even when the direct speed measurement is lost, the system will still have 3 sets of static(altitude and vertical speed), AoA , attitude, magnetic heading, turn rate, and 3D linear and angular acceleration to make all the calculations and crosschecks that you want, combined with the weight, CG and config information together with the FADEC data, to avoid that the system is responding to inaccurate data to cover for the scenario presented by 3we. You still can loose quite a bit of other things before you would not have enough data to keep this system working, at which point there would be no chance other than handling the controls to the pilot and God helps us.

    It really sounds so simple that I don't understand why it is not done, other than the manufacturer preferring a bigger risk of a deadly crash where the airline is liable rather than a very much smaller chance of a deadly crash where the manufacturer is liable.

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    Quote Originally Posted by 3WE View Post
    Gabieee asked a very specific question as to why P+P=P isn't used as a computerized speed calculation method. He also discussed autopilot disconnects.

    Based on ~75 personal experiences of watching P+P take a few minutes to settle on an airspeed, I offered the discussion that MABYE because it wasn't super accurate over the short term. What is your experience on using P+P=P to arrive at a target airspeed?
    So you prefer the human pilot to use P+P=P? Will the plane settle any faster?

    Plus...
    1- I don't necessarily agree that it takes so much to settle, especially taking into account that initially you will want mainly to maintain the performance tat you already had at the moment of loosing the speed indication, so you are already settled.

    2- When you want to change from P1+P1=P1 to P2+P2=P2 the transition happens witha perfromance (speed and vertical speed) that remains within P1 and P1. For example, if you want to transition from straight and level at 280 knots to -1000 FPM at 250 knots, you set the pitch and power for -1000 FPM at 250 knots and, whatever it takes, the plane will smoothly transition from level to -1000 FPM and from 280 knots to 250 knots. So what's the risk?

    3- Furthermore, the computer, much better than a human pilot, can make it more aggressive. In the previous example, the computer can idle the throttles and let the plane slow down to 250 knots while the autopilot smoothly increases the pitch to remain level and when approaching 250 knots the pitch can be let go down to keep the desired vertical speed while the throttle is increased to keep 250 knots. It doesn't get any better (or quicker) than that unless you start the descent by climbing to slow down faster.

    I love the concept of P+P=P being a powerful way to keep between stall and overspeed...I concede, it's not perfect for nailing a target speed. If there's a problem on the plane, follow procedures, but it's OK if you happen to know some fundamental stuff that's behind the procedures.

    Gabieee agreed.

    This is fundamental aeronautical stuff...I know you love to have it buried in cryptic acronyms in a computer, but the USA and forum polices allow discussion of that which you disdain.
    Yes, I agree conceptually. But the AF investigation revisited about a dozen of UAS events where, in 100% of them, the crews did not crash but did not either follow the prescribed P+P=P which I would almost say it would be still ok if you keep healthy traditional pitches and powers for the condition, except that all the interviewed crews declared that they had difficulty controlling the plane.

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    What are you talking about???

    You asked this:


    Quote Originally Posted by Gabriel View Post
    A good question could be why, to this day and to my knowledge, P+P=P has not been incorporated in the flight automation logic. It doesn't seem to be so complicated.
    I answered 'because maybe it takes a few minutes for the plane's speed to stabilize'.

    To which you asked aren't computers better than humans?

    That's a totally different question-but yes. (refer back a few posts where I say yes.)

    Then you say, so you prefer humans? Where the hell did that come from?

    Is this discussion or are you on a misplaced mission to tell the world that flight management systems MUST be redone everywhere to include P+P=P airspeed indications?

    If so, I'm the wrong person, and this forum is the wrong place. I'd prefer all that crap be left alone, INCLUDING the removal of MCAS... In the meantime, I'll leave it to the existing aeroengineers as to how pitot problems are handled by computers OR people.

    Please note- if you change pitch and power, it doesn't matter if a human or computer does it- the plane moves towards the new speed at the same rate and settles on it asymptotically (unless you tweak it) computer OR human. This may not be a very precise way to control airspeed- so maybe that's why your colleagues haven't invested in writing computer code?

    'Tweak' is key word. If the pitot is working things can be adjusted (over controlled if you will) to promptly establish a new speed. If it's not, you 'wait' through the asymptotic approach.
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    Quote Originally Posted by 3WE View Post
    Then you say, so you prefer humans? Where the hell did that come from?
    Please 3WE, from your last 5000 posts on the theme of scorn for systems and procedures? Not only do you prefer humans, you prefer improvising humans.

    And, we have a new insight into Evan...so if the system goes crazy and you don't have airspeed, you cannot choose a known and familiar power and pitch from memory,
    What Gabriel is proposing removes the opportunity for improvisation at that critical moment when a pilot's mind is most vulnerable to human factors. Even if such a system wasn't able to put you on a perfectly accurate speed, it would still be inarguably better than relying on pilots and their fundamental airmanship, as we have learned and learned and learned...

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    Quote Originally Posted by Evan View Post
    Blah blah blah...What Gabriel is proposing...blah blah blah...
    Cool bro'

    The best part is that nowhere in this thread did I say don't use systems or procedures. I did talk about fundamental aerodynamics. I guess your black and white mind twists that into "improvisation" because pilots aren't allowed to understand concepts like P+P=P.

    As I told Gabriel, if you want to propose a new system go for it. The best I can do to help you is talk to a few neighbors who work at Boeing...unfortunately, most of them deal with F-15/F-18 parts and stuff- not so much systems development. I also suspect they may be a little less interested in designing new automated stuff at this particular point and time.

    Bobby, ATL, could you forward this suggestion to your former and current colleagues and maybe it would work back to designers? It sounds cool.
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    Quote Originally Posted by 3WE View Post
    What are you talking about???

    You asked this:

    A good question could be why, to this day and to my knowledge, P+P=P has not been incorporated in the flight automation logic. It doesn't seem to be so complicated.
    I answered 'because maybe it takes a few minutes for the plane's speed to stabilize'.

    To which you asked aren't computers better than humans?

    That's a totally different question.
    How is that a totally different question? When you have an UAS event you have basically 3 choices: Automation relinquishes flight control to the human pilot and let them fly accordingly, automation keeps flight control and adapts to fly the plane accordingly, or nobody does anything and we are all along for the ride.

    If your answer "because maybe it takes a few minutes for the plane's speed to stabilize" is not done in that context, then how is that an answer to my original question? Say that it takes a few minutes for the plane's speed to stabilize. So what? Why has P+P=P not been incorporated in the flight automation logic? Yes, it takes some time for the speed to stabilize, and the sky is blue, and the water is wet. So what? I don't see a valid "because" there.

    'Tweak' is key word. If the pitot is working things can be adjusted (over controlled if you will) to promptly establish a new speed. If it's not, you 'wait' through the asymptotic approach.
    Of course, "pilot" above (the agent doing the "tweaking" or "overcontrol" can include Flesh or Otto. So we are back to square zero with the question.

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    Quote Originally Posted by 3WE View Post
    As I told Gabriel, if you want to propose a new system go for it. The best I can do to help you is talk to a few neighbors who work at Boeing...unfortunately, most of them deal with F-15/F-18 parts and stuff- not so much systems development. I also suspect they may be a little less interested in designing new automated stuff at this particular point and time.

    Bobby, ATL, could you forward this suggestion to your former and current colleagues and maybe it would work back to designers? It sounds cool.

    I'm the wrong person, and this forum is the wrong place.
    Do you mean that the industry didn't change their stall procedures because of the thread I started here on the subject?

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    Quote Originally Posted by Gabriel View Post
    ***context***
    Context? Indeed, context matters.

    Did you note (from your own words) that there is essentially ZERO use of P+P=P for automated compensation for pitot failure.

    How's that for context?

    Essentially NONE of your aeroengineer colleagues ever decided this was a proper thing to do...

    And then you ask why.

    "Because computers are good" doesn't answer that.

    Acknowledged: My answers were the purest of parlour-talk ass-hattery. But "computers being good" is not a reason why no one in the aerospace industry has seen the need for some sort of P+P=P Pitot-Failure Algorithm.

    Context? The clearest context is that you have developed some sort of fixation on computers and that is your answer to everything.

    Hey, I have a basic model new pickup truck...Chevy 1500 crew cab with a 6 cyl engine...I recently saw 30 MPG (60 MPH back roads with a tail wind). It drives nicely, pulls trailers OK (no not big ones), has ABS, and who knows what all engine controls- that run by computers...Yeah, computers are used in aeroplanies and they will be used MORE. But I got a beer that the Gabrielian P4FA routine ain't coming. I know you don't like my guessed explanation, but you asked the question. What's your answer as to why? All the other aeroengineers are stupid, unlike you?

    As I told Evan, is there anything specific you want me to tell my neighbors at Boeing to see if they can get the suggestion over to the development department? I repeat my concerns that they seem a bit disinterested regarding the subject of software modifications.




    As to another post above- where Evan yelled, "You do nothing" and assumed I'd disagree...I actually agree...Hell no, you absolutely do not do a full pull up...and the plane WAS flying fat dumb and happy [due to P+P=P no less] so don't screw with it.

    The key question there is if you are high up near coffin corner (or more blandly where your maximum speed and stall speed begin to get closer to each other)...maybe you want to do something about that since you don't have airspeed indications....well in that case, it might be wise to descend a bit (ATL crew actually suggested this). You can do it Evan's way and look up THE settings in THE book of acronymns, OR you can do it they way you have done every last departure from cruise altitude for the past 6 months. Whether you look it up in the book or chose something with which you are familiar- it is indeed based on the fundamental concept that P+P=P, but Evan doesn't like it when we refer to things in fundamental terms.



    Final thought- put your P4FA system in the CONTEXT that a plane is near coffin corner...Is +/- 10 knots good enough? +/- 5 knots? And again, what caused the pitot failure? Are there some modes where P4FA is going to do the wrong thing? Sitting here today, I can't imagine what that is, but a lot of smart people approved a stall-preventing-nose-over system for not having bad unintended consequences. I am guessing that aeroplanie designers have put lots of thought into how to handle Pitot Failures and designed what might be the most robust system to address it. Aggie ass-hat guess, but...
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