777 unreliable airspeed in cruise
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777 unreliable airspeed in cruise
Consider the following: you are flying at a high cruising level in your 777 and experience an AF447 kind of unreliable airspeed situation. According to Boeings memory items you should set 4 degrees of pitch and 70% N1 (at least in the GE model that is) and then consult the NNC which will lead you to the inflight performance tables for accurate values. Personally it doesn't feel quite natural to me and on my last type I was taught that if it flew straight an level for the past hours than it will for the next minute or so until the tables are handy.
Whats your (or your training departments) take on this?
Whats your (or your training departments) take on this?
A great deal of discussion in the archives of AF447 exists on this very thing. Although the 777 and A330 are both heavy twins, trying to compare the two (computerized) approaches to UAS is problematic, due to fundamental design discrepancies?
Has a 777 ever reacted (automatically) to UAS by altering (degrading) flight control to a separate flight control system? Climbed rapidly absent pilot input? Trimmed automatically into the STALL?
Has a 777 ever reacted (automatically) to UAS by altering (degrading) flight control to a separate flight control system? Climbed rapidly absent pilot input? Trimmed automatically into the STALL?
I'm told (different Boeing heavy type, but similar memory items) that we should follow the memory items regardless.
I too struggle with the concept of changing things if the aircraft is doing what it's supposed to be doing.
I too struggle with the concept of changing things if the aircraft is doing what it's supposed to be doing.
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Going by the book,
Unreliable airspeed in cruise will invite the necessary memory items FIRST. As unrealistic the values look, before jumping to the tables, it has to be concluded whether the airplane is in trim (still or gotten out of trim)
If in trim still, tables can be refered to with a/c flown in primary flight control mode with pitch/power etc.
If not in trim, the recent step added is - disconnection of PRIMARY FLIGHT COMPUTERS. (This enables a direct mode of operation of stabilizer and other flight controls)
Probably then jumping to the tables would be better.
In short -
Inducing a memory item value of QRH to a a/c already in a steady state won't be
Desirable as the a/c would probably be maintaining very accurate pitch/power however if things get out of hand due to changing airspeeds (& it most certainly will) then restrictimg to the memory item values to begin with won't be a bad idea.
That's just my opinion
corrections welcomed
Unreliable airspeed in cruise will invite the necessary memory items FIRST. As unrealistic the values look, before jumping to the tables, it has to be concluded whether the airplane is in trim (still or gotten out of trim)
If in trim still, tables can be refered to with a/c flown in primary flight control mode with pitch/power etc.
If not in trim, the recent step added is - disconnection of PRIMARY FLIGHT COMPUTERS. (This enables a direct mode of operation of stabilizer and other flight controls)
Probably then jumping to the tables would be better.
In short -
Inducing a memory item value of QRH to a a/c already in a steady state won't be
Desirable as the a/c would probably be maintaining very accurate pitch/power however if things get out of hand due to changing airspeeds (& it most certainly will) then restrictimg to the memory item values to begin with won't be a bad idea.
That's just my opinion
corrections welcomed
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A great deal of discussion in the archives of AF447 exists on this very thing. Although the 777 and A330 are both heavy twins, trying to compare the two (computerized) approaches to UAS is problematic, due to fundamental design discrepancies?
Has a 777 ever reacted (automatically) to UAS by altering (degrading) flight control to a separate flight control system? Climbed rapidly absent pilot input? Trimmed automatically into the STALL?
Has a 777 ever reacted (automatically) to UAS by altering (degrading) flight control to a separate flight control system? Climbed rapidly absent pilot input? Trimmed automatically into the STALL?
Agg, rather than over complicating a rather simple process, how about this.
Lets examine why the memory power and pitch settings are stipulated, and what they give you.
Realistically all they are designed to do is too keep you away from the ground until you can identify and manage your airspeed via alternative methods, they are in essence a basis from where you can start and are just like half the other memory items on the 777 in that they are as much about liability as they are to provide safety.
I KNOW that in the cruise 2.5 degrees and 80-85% N1 will yield level or near level flight within the airspeed limits, 4 degrees and 70% N1 is pointless, likewise down low, lets say below 10000ft or so, the numbers make more sense because they are designed to keep you away from the ground.
As for training departments, well where I work there is likely to be both a common sense approach as well as a slavish adherence depending upon the ego in the seat behind.
Lets examine why the memory power and pitch settings are stipulated, and what they give you.
Realistically all they are designed to do is too keep you away from the ground until you can identify and manage your airspeed via alternative methods, they are in essence a basis from where you can start and are just like half the other memory items on the 777 in that they are as much about liability as they are to provide safety.
I KNOW that in the cruise 2.5 degrees and 80-85% N1 will yield level or near level flight within the airspeed limits, 4 degrees and 70% N1 is pointless, likewise down low, lets say below 10000ft or so, the numbers make more sense because they are designed to keep you away from the ground.
As for training departments, well where I work there is likely to be both a common sense approach as well as a slavish adherence depending upon the ego in the seat behind.
There is a bulletin, subject: "avoidance of abrupt flight control input as a result of a sudden and unrealistic drop in indicated airspeed".
It reads as you wish in that IN THE ABSENCE OF EICAS MESSAGES Fly the airplane at normal pitch and power settings.
Hope this helps.
It reads as you wish in that IN THE ABSENCE OF EICAS MESSAGES Fly the airplane at normal pitch and power settings.
Hope this helps.
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Although our company is B737 iso B777 I'll give you our training department's vision (same philosophy for all Boeing models).
The logic of the checklist is the following:
1. Fly a safe pitch & power setting
2. Fly a known pitch & power setting
3. Compare the ASI with the known pitch & power settings to identifiy which ones are still reliable.
4. Use the reliable ASI
(This is a change to the old checklist, where the ASI's were compared with the other ASI's)
That being said, the checklist is written for all scenario's. We are being taught that some common sense does apply
If it is very obvious what is going on (like the cruise example above) you are actually already in step 2 so there is no use for the step 1 (memory items).
However, if there is any doubt or it is not immediately clear what is going on, buy yourself some time by applying the memory items. Then you can calmly investigate with the checklist.
The memory items are written in that way that at higher altitudes you will have a descent and at lower altitudes a shallow climb while always staying within the low & high speed limits.
The logic of the checklist is the following:
1. Fly a safe pitch & power setting
2. Fly a known pitch & power setting
3. Compare the ASI with the known pitch & power settings to identifiy which ones are still reliable.
4. Use the reliable ASI
(This is a change to the old checklist, where the ASI's were compared with the other ASI's)
That being said, the checklist is written for all scenario's. We are being taught that some common sense does apply
If it is very obvious what is going on (like the cruise example above) you are actually already in step 2 so there is no use for the step 1 (memory items).
However, if there is any doubt or it is not immediately clear what is going on, buy yourself some time by applying the memory items. Then you can calmly investigate with the checklist.
The memory items are written in that way that at higher altitudes you will have a descent and at lower altitudes a shallow climb while always staying within the low & high speed limits.
mop, you assume a 'hypothetical' situation, overlooking step one. How do you know airspeed is unreliable.
If only one EFIS airspeed display is suspect then perhaps a comparator alert is given.
If a background system is the problem, such as airdata, again there could be an alert or failure warning.
A more complex problem could involve several air data systems like AF, which challenged the automatic 'two out of three' comparator logic, and then handed the situation to the pilots to solve, - where the complexity of situation could 'fool' the two out of three pilots cross check ( but there were only two).
The first step is to determine what the problem is; a single or multiple airspeed display, or airdata which could involve many other aircraft systems - flight control, altitude, vs, ...
I suspect the checklist or memory items (I'm not T7 qualified) relates to specific circumstances. These conditions must be identified before acting. Check any explanation given by the manufacturer, is the malfunction alerted, if so how, what is the drill title, and what alternative actions are there.
Your scenario assumes a situation, then jumps to an activity which might be inappropriate for the actual situation.
Step '0', understand the situation, avoid expectation (manage surprise);
then apply the drill according to your current understanding, consider what should or could happen,
then reassess the situation according to your projections and revised actual conditions,
then reassess the situation, ...
then, ...
If only one EFIS airspeed display is suspect then perhaps a comparator alert is given.
If a background system is the problem, such as airdata, again there could be an alert or failure warning.
A more complex problem could involve several air data systems like AF, which challenged the automatic 'two out of three' comparator logic, and then handed the situation to the pilots to solve, - where the complexity of situation could 'fool' the two out of three pilots cross check ( but there were only two).
The first step is to determine what the problem is; a single or multiple airspeed display, or airdata which could involve many other aircraft systems - flight control, altitude, vs, ...
I suspect the checklist or memory items (I'm not T7 qualified) relates to specific circumstances. These conditions must be identified before acting. Check any explanation given by the manufacturer, is the malfunction alerted, if so how, what is the drill title, and what alternative actions are there.
Your scenario assumes a situation, then jumps to an activity which might be inappropriate for the actual situation.
Step '0', understand the situation, avoid expectation (manage surprise);
then apply the drill according to your current understanding, consider what should or could happen,
then reassess the situation according to your projections and revised actual conditions,
then reassess the situation, ...
then, ...
Can't speak for anyone else, but at some stage (like right now) it has sunk in that this discussion hosts large jet operators who don't know for sure what to do when they lose an instrument.... Haven't mustered the courage yet to step aboard an A330, let alone fly AirFrance....
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Concours, this thread scares me too.
The checklist is excellent. Just do it, and you will survive. The checklist will descend the aircraft. Which is what you want. Who wants to stay in coffin corner with unreliable airspeed? Not I, and not Boeing, that's for sure.
Ignoring a checklist in this circumstance could only ever be acceptable if following the checklist would further jeopardise safety. About the only example I could think of would be traffic below. If traffic below, simply turn off the airway and then follow the checklist.
If you were the F/O, and had an unreliable airspeed in the cruise, and the Captain said "I'm just going to set 2.5 degrees and 85%", what would you do?
Personally I would say, "No, there's a checklist for this", and "Maybe 2.5 degrees and 85% is correct, but will doing that improve safety? No? So let's follow the checklist.". If that didn't work it would be "Taking Over", and set the attitude and thrust from the QRH memory items.
Once the memory items are complete, we can reference the remainder of the checklist, and have a discussion about whether the Captain would like to resume being a Captain.
You see, when you start to vary SOP's, it can only be for a significant improvement in safety, and there are 2 pilots on that flight deck. They both need to be in the loop.
In both Air France and Air Asia, one pilot was doing the wrong thing, the other pilot knew it, but failed to take over properly.
The checklist is excellent. Just do it, and you will survive. The checklist will descend the aircraft. Which is what you want. Who wants to stay in coffin corner with unreliable airspeed? Not I, and not Boeing, that's for sure.
Ignoring a checklist in this circumstance could only ever be acceptable if following the checklist would further jeopardise safety. About the only example I could think of would be traffic below. If traffic below, simply turn off the airway and then follow the checklist.
If you were the F/O, and had an unreliable airspeed in the cruise, and the Captain said "I'm just going to set 2.5 degrees and 85%", what would you do?
Personally I would say, "No, there's a checklist for this", and "Maybe 2.5 degrees and 85% is correct, but will doing that improve safety? No? So let's follow the checklist.". If that didn't work it would be "Taking Over", and set the attitude and thrust from the QRH memory items.
Once the memory items are complete, we can reference the remainder of the checklist, and have a discussion about whether the Captain would like to resume being a Captain.
You see, when you start to vary SOP's, it can only be for a significant improvement in safety, and there are 2 pilots on that flight deck. They both need to be in the loop.
In both Air France and Air Asia, one pilot was doing the wrong thing, the other pilot knew it, but failed to take over properly.
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The unreliable airspeed checklist is great for when people are disorientated or unable to control their aircraft. However if you are in level flight at cruise and familiar with the pitch and attitude requirements of your aircraft and are aware that you have unreliable airspeed it is actually safer to leave the autopilot engaged and flying the aircraft level than to disengage it and raise the nose to a pitch attitude and power setting that will initially give you a climb followed by a descent. That is of course provided the aircraft is doing a satisfactory job. We are not familiar with hand flying at altitude and even flying the pitch and power settings as per the checklist will lead to a can of worms while one person hand flies it and the other goes through the checklist. I can understand that some modern pilots with no understanding of the basics need to stick slavishly to the checklist especially when things go awry but to deliberately take an aircraft away from a straight and level situation that is under control is not desirable.
Checklists cannot always be appropriate for every situation. For instance the ground proximity checklist dictates that the wings must be level for the manoeuvre however if I am heading for the highest point of a mountain I will bank left or right if necessary to avoid it rather than fly into it. It is best if some areas are left gray so pilots can follow checklists if appropriate or ignore it if slavish adherence to them can lead to jeopardising safety.
Checklists cannot always be appropriate for every situation. For instance the ground proximity checklist dictates that the wings must be level for the manoeuvre however if I am heading for the highest point of a mountain I will bank left or right if necessary to avoid it rather than fly into it. It is best if some areas are left gray so pilots can follow checklists if appropriate or ignore it if slavish adherence to them can lead to jeopardising safety.
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Draglift,
You state that if you are familiar with the pitch attitude and thrust in cruise then it is safer to leave the autopilot engaged.
Well, I hate to break it to you but the autopilot is not going to give you a pitch attitude. It's going to pitch for whatever pitch it thinks is necessary to maintain what it thinks is your altitude. Your altitude comes from the same air data computer that is giving you unreliable airspeed.
So which is it? Are you going to set a known pitch attitude and thrust, or are you going to leave the autopilot engaged? You can't do both. This is the danger of ignoring the checklist, even if your "non-modern aviator" ego thinks you can do better than the checklist. Look, maybe you can, maybe you can't. As soon as you choose to ignore the checklist the "modern" pilot sitting next to you now has no idea what is going on. So now you are single-pilot IFR with unreliable instruments. Not a nice place to be.
Yes, there can be a time and a place to decide that the checklist is not the most appropriate response to a situation. Those situations are defined as where following the checklist may jeopardize safety. This is not one of them.
Just set 4 degrees and 70%. It's safe. So what if you climb then descend? It will be a shallow climb followed by a shallow descent. It's a big sky out there. Then you are both in a known situation and the checklist will help you resolve the issue.
You state that if you are familiar with the pitch attitude and thrust in cruise then it is safer to leave the autopilot engaged.
Well, I hate to break it to you but the autopilot is not going to give you a pitch attitude. It's going to pitch for whatever pitch it thinks is necessary to maintain what it thinks is your altitude. Your altitude comes from the same air data computer that is giving you unreliable airspeed.
So which is it? Are you going to set a known pitch attitude and thrust, or are you going to leave the autopilot engaged? You can't do both. This is the danger of ignoring the checklist, even if your "non-modern aviator" ego thinks you can do better than the checklist. Look, maybe you can, maybe you can't. As soon as you choose to ignore the checklist the "modern" pilot sitting next to you now has no idea what is going on. So now you are single-pilot IFR with unreliable instruments. Not a nice place to be.
Yes, there can be a time and a place to decide that the checklist is not the most appropriate response to a situation. Those situations are defined as where following the checklist may jeopardize safety. This is not one of them.
Just set 4 degrees and 70%. It's safe. So what if you climb then descend? It will be a shallow climb followed by a shallow descent. It's a big sky out there. Then you are both in a known situation and the checklist will help you resolve the issue.
Last edited by Derfred; 21st Dec 2016 at 08:52.
I agree with Derfred.
Why is the first (recall) item on the checklist an instruction to disengage the autopilot? Because much of the automation has potentially become as unreliable as the airspeed. It follows up by removing misleading guidance and possible interference from the FBW systems.
There is significant danger involved in applying non-SOP methods due to instant diagnoses. There are many manifestations of pitot/static failures and part-failures and they can be even more confusing when they happen in turbulence and/or shear (which is unfortunate, as this is a more likely scenario). Best to just follow the QRH unless you’re actually going to hit something.
As an aside, I’ve often thought it would be useful to have a guarded button that removed all displays of airspeed from the cockpit and informed the automation to disregard it. Half of the annoyance is getting warnings and/or undesirable behaviour triggered by something you know is wrong.
Why is the first (recall) item on the checklist an instruction to disengage the autopilot? Because much of the automation has potentially become as unreliable as the airspeed. It follows up by removing misleading guidance and possible interference from the FBW systems.
There is significant danger involved in applying non-SOP methods due to instant diagnoses. There are many manifestations of pitot/static failures and part-failures and they can be even more confusing when they happen in turbulence and/or shear (which is unfortunate, as this is a more likely scenario). Best to just follow the QRH unless you’re actually going to hit something.
As an aside, I’ve often thought it would be useful to have a guarded button that removed all displays of airspeed from the cockpit and informed the automation to disregard it. Half of the annoyance is getting warnings and/or undesirable behaviour triggered by something you know is wrong.
Deferred, Im going to assume for a moment that you have many thousands of hours of experience, and more to the point a good deal of that time on the 777.
As we are talking about unreliable airspeed 'in the cruise" I'm curious to your rational that blindly adopting a pitch attitude and power setting without considering your situation is a sensible course of action..it is in fact on the basis of what your experience may be, troubling to me at the very least.
Deferred if you are genuinely advocating this, then I'd suggest a quick review of your Boeing QRH checklist instructions..contained within.
Seems to me that based on the document Boeing produces...they themselves provide some common sense guidance in this regard and even give an example to reiterate the point I believe.
Lets now assume for a moment you or your colleague aren't conscientious nor is your SA great and you find yourself close to max FMC alt when you encounter the top of a CB (at night..doesn't it always happen at night!) thanks to low radar reflectivity etc etc..and hey presto you get the stick shaker and the overspeed....are you going to reduce thrust to 70% and pitch up to 4 degrees? how long till you stall the aeroplane aerodynamically? how long till you overspeed it? how long till the other guy is useful again and gets past that startle effect?
I KNOW that 85% N1 and 2.5 degrees is safe in the cruise...and will keep me safe.
Don't believe that I'm not using a bit of common sense and experience to add resilience to the Boeing memory items and checklist?...then lets look past the memory items...and understand where the rest of the checklist leads you.
So there we have it...Pitch and N1 are reliable! who'd have thought it?
moving on...
And we get get to the meaty bit...because we've ALREADY established a safe pitch and power setting for the current phase of flight, it gives us more time to make a better assessment of the issue thanks to the aircraft never entering an undesired state.
Now we have time to complete the checklist having overcome that startle effect and dealt with the fear factor, after all its an aeroplane and basic physics apply, Attitude, Power, Trim.....just like a C150.
The intent of checklists and memory items are to AID to the pilot, they are and were never intended to replace your thought and reasoning process.
Lastly, failing all of that merely take a quick mental note of your steady state N1 and Attitude when it settles down in the cruise...you'd be amazed how that works.
Now if we are discussing unreliable airspeed in the climb or descent...thats another topic for discussion, and much more related and in tune to the checklist memory items for N1 and Attitude.
As we are talking about unreliable airspeed 'in the cruise" I'm curious to your rational that blindly adopting a pitch attitude and power setting without considering your situation is a sensible course of action..it is in fact on the basis of what your experience may be, troubling to me at the very least.
Deferred if you are genuinely advocating this, then I'd suggest a quick review of your Boeing QRH checklist instructions..contained within.
While every attempt is made to supply needed non–normal checklists, it is not possible to develop checklists for all conceivable situations. In some smoke, fire, or fumes situations, the flight crew may need to move between the Smoke, Fire or Fumes checklist and the Smoke or Fumes Removal checklist
Lets now assume for a moment you or your colleague aren't conscientious nor is your SA great and you find yourself close to max FMC alt when you encounter the top of a CB (at night..doesn't it always happen at night!) thanks to low radar reflectivity etc etc..and hey presto you get the stick shaker and the overspeed....are you going to reduce thrust to 70% and pitch up to 4 degrees? how long till you stall the aeroplane aerodynamically? how long till you overspeed it? how long till the other guy is useful again and gets past that startle effect?
I KNOW that 85% N1 and 2.5 degrees is safe in the cruise...and will keep me safe.
Don't believe that I'm not using a bit of common sense and experience to add resilience to the Boeing memory items and checklist?...then lets look past the memory items...and understand where the rest of the checklist leads you.
5 Chooseone:
Aircraft can be trimmed to desired pitch attitude:
Go to step 6
Aircraft cannot be trimmed to desired pitch
attitude:
PRIMARY FLIGHT COMPUTERS disconnectswitch.............. DISC
Go to step 6
6 The following are reliable:
Attitude
N1
Ground speed Radio altitude
Aircraft can be trimmed to desired pitch attitude:
Go to step 6
Aircraft cannot be trimmed to desired pitch
attitude:
PRIMARY FLIGHT COMPUTERS disconnectswitch.............. DISC
Go to step 6
6 The following are reliable:
Attitude
N1
Ground speed Radio altitude
moving on...
Note: Stick shaker, overspeed warning, and AIRSPEED LOW alerts may sound erroneously or simultaneously.
Note: The flight path vector, and pitch limit indicator are unreliable.
7
Refer to the Flight with Unreliable Airspeed table in the Performance Inflight chapter and set the pitch attitude and thrust setting for the current airplane configuration and phase of flight.
Note: The flight path vector, and pitch limit indicator are unreliable.
7
Refer to the Flight with Unreliable Airspeed table in the Performance Inflight chapter and set the pitch attitude and thrust setting for the current airplane configuration and phase of flight.
Now we have time to complete the checklist having overcome that startle effect and dealt with the fear factor, after all its an aeroplane and basic physics apply, Attitude, Power, Trim.....just like a C150.
The intent of checklists and memory items are to AID to the pilot, they are and were never intended to replace your thought and reasoning process.
Lastly, failing all of that merely take a quick mental note of your steady state N1 and Attitude when it settles down in the cruise...you'd be amazed how that works.
Now if we are discussing unreliable airspeed in the climb or descent...thats another topic for discussion, and much more related and in tune to the checklist memory items for N1 and Attitude.
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Personally I would say, "No, there's a checklist for this", and "Maybe 2.5 degrees and 85% is correct, but will doing that improve safety? No? So let's follow the checklist.". If that didn't work it would be "Taking Over", and set the attitude and thrust from the QRH memory items.
Once the memory items are complete, we can reference the remainder of the checklist, and have a discussion about whether the Captain would like to resume being a Captain.
Once the memory items are complete, we can reference the remainder of the checklist, and have a discussion about whether the Captain would like to resume being a Captain.
- Daylight CAVOK
- FL370 for the last hour
Pitch and thrust have remained the same for the last hour. You get a warning: "Airspeed disagree". The aircraft does not change pitch & thrust.
You look and see that the ASI are:
- PF: 253kts
- SBY: 250kts
- PM: 45kts
After crew agreement, the crew realizes that it's pretty obvious what is going on and just remains at the known pitch settings while doing the checklist...
Now, if you would be my copilot on that day and you don't agree with this. Then sure, we'll do the memory items. That would fall under the "not immediately obvious case" (at least for 1 pilot). I agree with you that the last thing you want is a flight deck where 1 of the pilots is lost.
Anyway, scenario 2: During descent (IMC), you hear a bang. 10 seconds later you get "Airspeed unreliable". The A/P starts pitching down. You decide together that you do the memory items...
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Can't speak for anyone else, but at some stage (like right now) it has sunk in that this discussion hosts large jet operators who don't know for sure what to do when they lose an instrument.... Haven't mustered the courage yet to step aboard an A330, let alone fly AirFrance....
It is what it is...
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What does Boeing say? 4 NU and 70%, then checklist.
If you know the numbers and can instantly match what the checklist requires??? The checklist has 5,000' altitude blocks and scans 100,000 lbs. What's the checklist going to get you? After extrapolating from four boxes?? It's trying to get you the pitch and power you've been holding for the last 30 minutes. But if you don't know it don't start guessing.
Just checked what I see vs. what the checklist calls for. .8% difference. Any guesses on if a four box extrapolated answer, with heart pounding, will be within .8%???
If you know the numbers and can instantly match what the checklist requires??? The checklist has 5,000' altitude blocks and scans 100,000 lbs. What's the checklist going to get you? After extrapolating from four boxes?? It's trying to get you the pitch and power you've been holding for the last 30 minutes. But if you don't know it don't start guessing.
Just checked what I see vs. what the checklist calls for. .8% difference. Any guesses on if a four box extrapolated answer, with heart pounding, will be within .8%???