B777: No Overbank After Flameout

WarrenPlatts · Fri Mar 25, 2016 6:44 pm

It is well known that after both engines on a Boeing 777 flameout due to fuel exhaustion that the autopilot will shut off. However, I have been told that the overbank protection also shuts off? Is this true?!?

77west · Fri Mar 25, 2016 8:46 pm

Quoting WarrenPlatts (Thread starter):
It is well known that after both engines on a Boeing 777 flameout due to fuel exhaustion that the autopilot will shut off. However, I have been told that the overbank protection also shuts off? Is this true?!?

If the flight computers go into alternate mode, then yes. But upon the APU and or RAT coming online, some of those protections are restored.

AIRWALK · Fri Mar 25, 2016 9:06 pm

If you lose engines and APU to fuel exhaustion, you only have the RAT and battery to provide electrical power. If this happens you will go to secondary flight control mode where you lose your envelope protection

WarrenPlatts · Fri Mar 25, 2016 9:53 pm

So simulators that have the a/c doing 90+ degree banking upon flameout (assuming no controlled flight inputs) are actually accurate? No software bugs?

Also: Is it the case that RAT is not enough to deploy flaps?

7BOEING7 · Sat Mar 26, 2016 5:04 am

Quoting WarrenPlatts (Reply 3):
So simulators that have the a/c doing 90+ degree banking upon flameout (assuming no controlled flight inputs) are actually accurate? No software bugs?

Simulators are not necessarily programed to provide accurate representations in this case. Take your hands off the controls in level flight in a simulator the airplane will stay in level flight, the real airplane won't.

Quoting WarrenPlatts (Reply 3):
Also: Is it the case that RAT is not enough to deploy flaps?

The RAT will provide hydraulic power for the primary flight controls not he flaps (hydraulically or electrically).

WarrenPlatts · Sat Mar 26, 2016 4:41 pm

Gentlemen, thanks for the replies. Sounds like it is uncertain as to exactly what would happen to the overbank protection if a B777 were to run out of fuel.

AIRWALK · Sat Mar 26, 2016 5:24 pm

Quoting WarrenPlatts (Reply 5):
Sounds like it is uncertain as to exactly what would happen to the overbank protection if a B777 were to run out of fuel.

You will lose envelope protection (overbank, overspeed, stall) if both engines flame out after running out of fuel

ThrottleHold · Sat Mar 26, 2016 7:16 pm

You can do a 90 degree bank during normal flight if you want. The protections will fight you, but it'll let you do it.

7BOEING7 · Sat Mar 26, 2016 10:06 pm

Quoting ThrottleHold (Reply 7):
You can do a 90 degree bank during normal flight if you want. The protections will fight you, but it'll let you do it.

Or disengage the autopilot via the A/P DISENGAGE bar on the MCP and the overbank protection is gone.

WarrenPlatts · Sat Mar 26, 2016 10:17 pm

Quoting WarrenPlatts (Reply 5):
You will lose envelope protection (overbank, overspeed, stall) if both engines flame out after running out of fuel

Sir, I do not doubt you, but is your statement based on simulator results, or are there independent reasons for believing that?

Quoting ThrottleHold (Reply 7):
You can do a 90 degree bank during normal flight if you want. The protections will fight you, but it'll let you do it.

I respectfully ask the same question here as well: I mean has anyone really ever tried this with an actual B777?

Florianopolis · Sun Mar 27, 2016 4:48 am

Quoting WarrenPlatts (Reply 3):
So simulators that have the a/c doing 90+ degree banking upon flameout (assuming no controlled flight inputs) are actually accurate?

Quoting 7BOEING7 (Reply 4):
Simulators are not necessarily programed to provide accurate representations in this case. Take your hands off the controls in level flight in a simulator the airplane will stay in level flight, the real airplane won't.

I thought simulators (at least the big expensive ones) were actually pretty accurate in the first-principles lateral and directional stability (or instability) that determine how a plane will stay straight+level+coordinated or not without control input...such as develop a nasty dutch roll, or spiral into the ground.

You cannot certify an airplane that hands pilots an aerodynamically unstable airplane if the FBW or augmentation turns off. That said, I think most modern airliners tend towards dutch roll stability at the expense of spiral stability (basically a trade between vertical fin size and wing dihedral, oversimply). So if the 777 is that way, it would eventually spiral into an overbank and overspeed condition into the ground.

Quoting WarrenPlatts (Reply 9):
I respectfully ask the same question here as well: I mean has anyone really ever tried this with an actual B777?

If Boeing didn't test it explicitly in flight tests, they tested enough data points around it to understand 100% what would happen in that scenario.

7BOEING7 · Sun Mar 27, 2016 4:54 am

Quoting WarrenPlatts (Reply 9):
Sir, I do not doubt you, but is your statement based on simulator results, or are there independent reasons for believing that?

Having turned off all four engine generators several hundred times, I will state that you end up in secondary mode and per the ops manual, in secondary mode envelope protection is not available.

Quoting WarrenPlatts (Reply 9):
I respectfully ask the same question here as well: I mean has anyone really ever tried this with an actual B777?

Although I haven't gone to 90 degrees, I have gone to 40+/- degrees several hundred times also and can attest to the forces encountered in the process.

mmo · Sun Mar 27, 2016 8:17 am

Quoting WarrenPlatts (Reply 9):
Sir, I do not doubt you, but is your statement based on simulator results, or are there independent reasons for believing that?

I can assure you the simulators are programmed and certified to replicate all of the aircraft characteristics in the normal flight envelope. Having a dual engine flameout is in the normal envelope. As such the such, the simulator will replicate the aircraft. Where the simulator has problems is when the aircraft is out of the "normal envelope" so, extreme unusual attitudes are very difficult to simulate in the sim, a loop may be able to be performed in the sim but it will not exhibit the same characteristics the aircraft would.

Quoting WarrenPlatts (Reply 9):
I respectfully ask the same question here as well: I mean has anyone really ever tried this with an actual B777?

I have been to 60 degrees and it can be done in the aircraft.

zeke · Sun Mar 27, 2016 9:22 am

Quoting WarrenPlatts (Reply 3):

So simulators that have the a/c doing 90+ degree banking upon flameout (assuming no controlled flight inputs) are actually accurate? No software bugs?

A simulator, being the term used by regulators, manufacturers, and operators to mean a certified device used for training of pilots will not do that.

What a PC program does on your home computer does is a different matter, it is not a simulator.

Quoting ThrottleHold (Reply 7):

You can do a 90 degree bank during normal flight if you want. The protections will fight you, but it'll let you do it.

Sorry, 90 AOB is not possible during "normal flight", that is way past the certified envelope and puts the aircraft into "experimental flight".

The manufacturer will not even try that during experimental flight testing.

apfpilot · Mon Mar 28, 2016 3:16 pm

Has anyone in this thread tried to do a spin in a level D simulator? I haven't had the opportunity to use a simulator that advanced but I have "flown" some of the lower fidelity level A sims for some citations and never could get one to break into a spin. My guess is that the flight modeling isn't advanced enough in those types of devices to handle something that far out of the envelope?

WarrenPlatts · Mon Mar 28, 2016 3:31 pm

Quoting 7BOEING7 (Reply 11):
Having turned off all four engine generators several hundred times, I will state that you end up in secondary mode and per the ops manual, in secondary mode envelope protection is not available.

Quoting Florianopolis (Reply 10):
So if the 777 is that way, it would eventually spiral into an overbank and overspeed condition into the ground.

Quoting mmo (Reply 12):
extreme unusual attitudes are very difficult to simulate in the sim, a loop may be able to be performed in the sim but it will not exhibit the same characteristics the aircraft would.

Quoting zeke (Reply 13):
Sorry, 90 AOB is not possible during "normal flight", that is way past the certified envelope and puts the aircraft into "experimental flight". The manufacturer will not even try that during experimental flight testing.

OK, there seems to be a consensus that after a flameout, that (1) not only does the autopilot cease to function, but envelope protection (overbank, overspeed, stall) becomes unavailable, (2) the a/c would bank over and enter an overspeed, overbanked spiral dive; but that (3) in extreme "experimental flight" conditions, professional simulators may not generate results that would happen in the real world.

Thus, the question is what happens after a flameout: I'm thinking specifically of Mike Exner's experiments with the most advanced simulators where he attempted to simulate a flameout with zero controlled flight inputs, and the results he got were very extreme, experimental flight conditions: banking actually exceeding 90 degrees, and speeds very nearly equal to Mach 1. Thus, in your all's considered, professional opinions: (a) are such bank angles and speeds to be expected at all? and (b) given severe g forces that 7BOEING7 alluded to at much lower speeds and bank angles, could a B777 withstand 90+ degree banking at virtual Mach 1 speeds with out breaking up in midflight?

[Edited 2016-03-28 08:33:32]

intsim · Mon Mar 28, 2016 3:40 pm

I was looking for the max operating speed for the RAT on the 777. I was thinking about the partial handshake and if that was the RAT deploying and breaking loose from the airframe. I found some items about what it powers and what deploys it but not max speeds. Thank you for any input.

BoeingGuy · Mon Mar 28, 2016 3:50 pm

Quoting zeke (Reply 13):
Sorry, 90 AOB is not possible during "normal flight", that is way past the certified envelope and puts the aircraft into "experimental flight".

The manufacturer will not even try that during experimental flight testing.

Sure they do. Sometimes during high bank angle stall testing, rather than pushing over hard during the recovery and making everyone sick, they will roll up to around 90 degrees and let the nose drop back to level. I know that was done recently on a certain test flight.

Also the first 777 was unintentionally at 110 degrees bank angle during stall testing.

Quoting mmo (Reply 12):

Quoting WarrenPlatts (Reply 9):
I respectfully ask the same question here as well: I mean has anyone really ever tried this with an actual B777?

I have been to 60 degrees and it can be done in the aircraft.

Bank Angle Protection (BAP) on the 777 and 787 use the Backdrive Actuators to move the control wheel in the opposite direction of the bank when bank angle exceeds 35 degrees. I don't want to start and A vs B argument, but it's designed so the pilots have ultimate authority of the airplane. Thus, they can easily override it if necessary.

The Backdrive Actuators are what backdrives the controls on the 777 and 787 (e.g. Fly-by-wire airplanes).

BAP doesn't work in the Secondary or Direct fly-by-wire modes. If my memory serves me correct, when both engines fail you lose all Pitot Heat due to the electrical architecture. That drops you into the Secondary Mode, which makes the Autopilot and BAP (and stall and overspeed protection) unavailable. You would still have some Yaw Damper capability.

Maybe someone with more current experience can verify if my recollection that a dual engine failure results in loss of Pitot Heat is accurate.

Quoting 7BOEING7 (Reply 8):
Or disengage the autopilot via the A/P DISENGAGE bar on the MCP and the overbank protection is gone.

Yes, but you don't even have to disconnect the autopilot. Anytime you pull down the white bar on the MCP, the autopilot and BAP are unavailable.

The white bar on the MCP is a "positive disconnect", say if your control wheel autopilot disconnect switches failed. On the 777 and 787, pulling down the white bar removes power from the Backdrive Actuators. Thus, you wouldn't have Bank Angle Protection (BAP).

zeke · Mon Mar 28, 2016 5:39 pm

Quoting BoeingGuy (Reply 17):
Sure they do. Sometimes during high bank angle stall testing, rather than pushing over hard during the recovery and making everyone sick, they will roll up to around 90 degrees and let the nose drop back to level. I know that was done recently on a certain test flight.

Also the first 777 was unintentionally at 110 degrees bank angle during stall testing.

That is completely different, read what I replied to "You can do a 90 degree bank during normal flight if you want. The protections will fight you, but it'll let you do it.". The reason for that is you will have a 100% increase in stall speed and pull 4.4g at around 75 deg AoB, anything past 67 degrees you outside the certified limits.

Boeing (nor does Airbus) does that sort of testing. What you are talking about is a fairly standard recovery technique where large bank angle is helpful in reducing excessively high pitch attitudes, but you are not pulling high g. I talked about that in some detail on the AF447 threads as the correct recovery technique.

Florianopolis · Mon Mar 28, 2016 6:38 pm

Before this thread goes off the deep end, can we be sure we're answering the same question?

Quoting WarrenPlatts (Reply 15):
Thus, the question is what happens after a flameout:

Presumably, the 777 RAT would deploy, and (this is speculation) you'd have some electrical power to the flight computers and thus some control augmentation enabling wing leveling, yaw damper, or other automatic trimming. If so, no control input would be required to keep the airplane going in a wings-level, coordinated flight path.

If no stability augmentation (e.g., FBW, yaw damper, wing leveler, over-bank protection) came back, and you didn't touch the controls, what would happen?

I believe the answer to that question is: Any modern airliner will probably have more directional stability than lateral stability, and will eventually spiral into the ground.

In re the flight simulator fidelity, here's a decent article on the subject:
http://aviationweek.com/awin/upset-recovery-sims

7BOEING7 · Mon Mar 28, 2016 11:31 pm

Quoting Florianopolis (Reply 19):
Presumably, the 777 RAT would deploy, and (this is speculation) you'd have some electrical power to the flight computers and thus some control augmentation enabling wing leveling, yaw damper, or other automatic trimming. If so, no control input would be required to keep the airplane going in a wings-level, coordinated flight path.

You're still in Secondary mode, the yaw damper may or may not be operative, TAC is not operative and in the 777 there is no "wing leveling". STRIKE1

Quoting Florianopolis (Reply 19):
If no stability augmentation (e.g., FBW, yaw damper, wing leveler, over-bank protection) came back, and you didn't touch the controls, what would happen?

Even in the most perfectly trimmed (manually) 777 one wing or the other will roll off and eventually (without protections) you will go out of control. STRIKE 2

And finally if you don't have a simultaneous dual engine flameout (MH370 scenario - autopilot/autothrottle on, hdg select, alt hold or similar), when the second engine does flameout the good engine will go to max power to maintain airspeed, the TAC will drive the rudder as far as it can but won't be enough to hold heading and the airplane will start banking. When the second engine flames out you're already in a turn and its downhill from there. STRIKE 3
(Zeke will probably be all over me for this)

Florianopolis · Tue Mar 29, 2016 12:50 am

Quoting 7BOEING7 (Reply 20):
And finally if you don't have a simultaneous dual engine flameout

Oooh, that's something I hadn't thought about. I was imagining a hypothetical scenario where someone just turned everything off, or pulled circuit breakers all at once. Thrust asymmetry would ruin everything.....

Slightly off topic -- I'm still not sure why the rudder in AirAsia 8501 had an uncommanded deflection when the FAC was turned off.

I'll put my asbestos suit on now....

BoeingGuy · Tue Mar 29, 2016 4:20 am

Quoting 7BOEING7 (Reply 20):

And finally if you don't have a simultaneous dual engine flameout (MH370 scenario - autopilot/autothrottle on, hdg select, alt hold or similar), when the second engine does flameout the good engine will go to max power to maintain airspeed, the TAC will drive the rudder as far as it can but won't be enough to hold heading and the airplane will start banking. When the second engine flames out you're already in a turn and its downhill from there. STRIKE 3

I would question that. In this case, the remaining engine probably goes to the CRZ thrust limit (I forget if you have to manual select the CON limit on the 777 or it the TMCF does so automatically when one engine fails). It doesn't go to "max power".

You are saying that TAC and the available autopilot aileron input doesn't have enough authority to maintain straight and level flight with one engine at max cruise thrust? I dispute that.

Remember the autopilot won't disconnect until the PFCs drop to the Secondary Mode.

zeke · Tue Mar 29, 2016 8:54 am

Quoting 7BOEING7 (Reply 20):
(Zeke will probably be all over me for this)

My

A flameout is not a failure, so with forward speed, and engine that has flamed out will still be able to drive the accessories, i.e an engine that has flamed out at cruise speed may still be able to drive the hydraulics and electrics.

It is likely that for a fuel starvation case, there will be brief relights as fuel makes it way to the collector tanks from within the wing.

Upon double engine flame-out, the APU will attempt to start, and may run for some time with the fuel in the line.

Realistically I dont see a scenario where the aircraft will stop trying to relight, and I would think it would get a few automatic relights in during a descent.

7BOEING7 · Tue Mar 29, 2016 4:54 pm

Quoting zeke (Reply 23):
A flameout is not a failure, so with forward speed, and engine that has flamed out will still be able to drive the accessories, i.e an engine that has flamed out at cruise speed may still be able to drive the hydraulics and electrics.

On the 777 with a left engine flameout you'll lose the generators on that side but the right generator will pick up the left main bus and the right backup generator will pick up the left transfer bus so everything is "normal". As for hydraulics you may or may not lose the EDP but that's only one of many pumps.

Quoting zeke (Reply 23):
It is likely that for a fuel starvation case, there will be brief relights as fuel makes it way to the collector tanks from within the wing.

There will be continuous relight attempts thru the autostart system but IMHO there will not be enough fuel to get an engine back to idle.

Quoting zeke (Reply 23):
Upon double engine flame-out, the APU will attempt to start, and may run for some time with the fuel in the line.

It will probably start but I don't think it will last very long.

Quoting BoeingGuy (Reply 22):
You are saying that TAC and the available autopilot aileron input doesn't have enough authority to maintain straight and level flight with one engine at max cruise thrust? I dispute that.

Initially there is enough authority but TAC does not fully compensate for a failed engine (per the ops manual) and has the airspeed decays (maintaining altitude) more and more aileron will be required to maintain wings level. Even if you can maintain wings level the minute the other engine quits you'll have rudder a lot one way, control wheel returning to neutral and very few protecions -- the airplane will go out of control.

[Edited 2016-03-29 10:22:02]

WarrenPlatts · Wed Mar 30, 2016 3:42 pm

Quoting ThrottleHold (Reply 7):
You can do a 90 degree bank during normal flight if you want. The protections will fight you, but it'll let you do it.

Quoting 7BOEING7 (Reply 11):
Although I haven't gone to 90 degrees, I have gone to 40 /- degrees several hundred times also and can attest to the forces encountered in the process.]

Quoting mmo (Reply 12):
I have been to 60 degrees and it can be done in the aircraft.

This is interesting and leads to a question that is slightly off topic. There is that infamous primary radar plot of MH370 that shows that has one plotted position way off to the right of the average track, that led to speculation that the aircraft was "juking".

I did some back of the envelope calculations and found (if I did the math right) that it would take a bank angle of like 75+ degrees--which is much more than the overbank protection. Thus, I concluded the maneuver must have been impossible, and that the radar bearing must have been in error.

However, from what you guys are telling me, the overbank protection can be easily overridden. Assuming level flight, such bank angles would require pulling g's approaching a load factor of 5. That would barely be within the physiologically possible envelope, assuming the aircraft itself could withstand a load factor of ~5.*

In your all's considered, professional opinion, would you agree that such a maneuver is possible in principle in real life?

* According to the formula n = 1/cos(B.A.), I get the following load factors:

75.0° 3.9
75.5° 4.0
76.0° 4.1
76.5° 4.3
77.0° 4.4
77.5° 4.6
78.0° 4.8
78.5° 5.0
79.0° 5.2
79.5° 5.5
80.0° 5.8

WarrenPlatts · Tue Apr 05, 2016 12:10 pm

OTOH, I guess if one wanted to lose a lot of altitude in a big hurry, and yet stay on the same track, probably it would be best to do a diving bank to the right, say, then left, and then right again--as opposed to pushing the nose straight down and experiencing negative g-forces?

mmo · Tue Apr 05, 2016 1:44 pm

Quoting WarrenPlatts (Reply 26):
nd then right again--as opposed to pushing the nose straight down and experiencing negative g-forces?

The "recommended technique" is simply to start a turn to the side and on swept wing aircraft that will allow the nose to drop below the horizon and the descent will begin. There is really no need to go left and right. Once your nose is below the horizon it becomes a very controlled maneuver.

WarrenPlatts · Sun Apr 10, 2016 12:57 pm

I found this manual that explains the overbank "philosophy" (thanks to RetiredF4 at jeffwise.net).

Because the system is controlled electronically, there is an opportunity to include system control augmentation and envelope protection features that would have been difficult to provide in a conventional mechanical system. The 777 Primary Flight Control System has made full use of the capabilities of this architecture by including such features as:

• Bank angle protection
• Turn compensation
• Stall and overspeed protection
• Pitch control and stability augmentation
• Thrust asymmetry compensation

More will be said of these specific features later. What should be noted, however, is that none of these features limit the action of the pilot. The 777 design utilizes envelope protection in all of its functionality rather than envelope limiting. Envelope protection deters pilot inputs from exceeding certain predefined limits but does not prohibit it. Envelope limiting prevents the pilot from commanding the airplane beyond set limits. For example, the 777 bank angle protection feature will significantly increase the wheel force a pilot encounters when attempting to roll the airplane past a predefined bank angle. This acts as a prompt to the pilot that the airplane is approaching the bank angle limit. However, if deemed necessary, the pilot may override this protection by exerting a greater force on the wheel than is being exerted by the backdrive actuator. The intent is to inform the pilot that the command being given would put the airplane outside of its normal operating envelope, but the ability to do so is not precluded. This concept is central to the design philosophy of the 777 Primary Flight Control System.

http://www.davi.ws/avionics/TheAvionicsHandbook_Cap_11.pdf

WarrenPlatts · Mon Apr 18, 2016 12:16 pm

OK, just a couple more questions, and then I'll stop bugging you guys. I have just reread the thread, and there seems to be an ambiguity: I get it that if only the RAT is running, there is no envelope protection. But what if, after flameout of main engines, the APU is running? Is there overbank protection if the APU is running?

The reason I'm asking is because in at least one of Mr. Exner's simulations, after flameout, the aircraft spiraled at a 35 degree bank angle for 5 minutes, and then, suddenly, the bank angle became much more pronounced. It seems to my non-expert eyes that there would be fuel in the lines to run the APU for a while, thus accounting for the 35 degree bank angle; when the APU ran out of fuel, and only RAT is running, that would explain the steeper bank angles.

Thanks!

BoeingGuy · Mon Apr 18, 2016 5:36 pm

Quoting WarrenPlatts (Reply 29):
I get it that if only the RAT is running, there is no envelope protection. But what if, after flameout of main engines, the APU is running? Is there overbank protection if the APU is running?

It isn't dependent on whether the RAT or APU is running. It's dependent on what Fly-By-Wire mode you are in. The APU does not directly correlate to whether Bank Angle Protection is functioning, for example.

If you are in Normal mode, you have full envelope protections and the Autopilot is available. If you are in Secondary or Direct Mode, you have lost envelope protections and the autopilot is unavailable (IIRC, you do have some Yaw Damper functionality available in Secondary Mode).

Direct Mode only occurs if your Primary Flight Computers fail, so we'll assume we aren't in the that mode for your question. Loss of primary air data or inertial inputs drop you into Secondary Mode. IIRC, if both engines fail you lose your Pitot Heat and that will cause you to do into Secondary Mode. Anyone have more current info on this?

If you lose both engines, the RAT will also automatically drop and your APU will automatically start.

The only scenario I can think of off-hand where only the RAT is providing electrical power is fuel starvation. The only scenario I can think of where only the APU and RAT are running is dual engine failure, or both engine driven generators and both backup generators failed.

OldAeroGuy · Mon Apr 18, 2016 7:27 pm

If both engines are inoperative (particularly if it's due to fuel exhaustion), losing bank angle protection should be low on the pilots' list of primary concerns.

7BOEING7 · Mon Apr 18, 2016 11:29 pm

Quoting WarrenPlatts (Reply 29):
But what if, after flameout of main engines, the APU is running? Is there overbank protection if the APU is running?

With the loss of both engines you drop into Secondary mode as indicated in Reply 30 by BoeingGuy and the only way to get back to Normal mode is to select the Primary Flight Controls disconnect switch to DISC then back to AUTO -- it won't go back to Normal mode just because there's electrical power.

Quoting BoeingGuy (Reply 30):
The only scenario I can think of off-hand where only the RAT is providing electrical power is fuel starvation.

True, however in a scenario where the RAT drops and the APU actually starts and stays running, the RAT continues to power some buses until the airspeed drops below about 80kts (?) on the runway (hopefully).

mandala499 · Tue Apr 19, 2016 2:34 pm

Quoting WarrenPlatts (Reply 26):
I guess if one wanted to lose a lot of altitude in a big hurry, and yet stay on the same track, probably it would be best to do a diving bank to the right, say, then left, and then right again--as opposed to pushing the nose straight down and experiencing negative g-forces?

Sideslip... left bank with right rudder, or right bank with left rudder, will result in a much faster descent rate for a given pitch than if with wings level...

Quoting WarrenPlatts (Reply 29):
But what if, after flameout of main engines, the APU is running? Is there overbank protection if the APU is running?

If you want overbank protections while on APU power, you switch to the APU first before the engines flame out or shutdown... Once you go to secondary mode due to the flameouts... the secondary mode if I remember correctly latches on until you're on the ground.

And when was the last time we had dual engine flameout on a 777 and the pilots survived? BA038?

BoeingGuy · Tue Apr 19, 2016 3:47 pm

Quoting mandala499 (Reply 33):
Once you go to secondary mode due to the flameouts... the secondary mode if I remember correctly latches on until you're on the ground.

I don't believe that's correct. If you cycle the Primary Flight Computer switch, it will select the highest available mode. If you are in Secondary, but the system is able to be in Normal and you cycle the PFC switch, it will go back to Normal.

Quoting OldAeroGuy (Reply 31):
If both engines are inoperative (particularly if it's due to fuel exhaustion), losing bank angle protection should be low on the pilots' list of primary concerns.

We all know that. This was an academic question. The OP was interested in this topic.

7BOEING7 · Tue Apr 19, 2016 4:55 pm

Quoting mandala499 (Reply 33):
the secondary mode if I remember correctly latches on until you're on the ground.

See Reply 32

mandala499 · Tue Apr 19, 2016 6:34 pm

Quoting 7BOEING7 (Reply 35):
See Reply 32

and

Quoting BoeingGuy (Reply 34):
but the system is able to be in Normal and you cycle the PFC switch, it will go back to Normal.

Thanks... my question the becomes, what is required for it to be in Normal mode?
1. ADIRU input (obviously)
2. HYDs (electric demand pumps adequate instead of the primary pumps?)
3. Electricals (for pitot heat, ADIRU power, Hyd elec pumps)...
What else?

For the electricals, if the APU can be on, it can power both the AC main buses so it should be business as usual on that end right?

skyhawkmatthew · Wed Apr 20, 2016 1:20 am

Quoting mandala499 (Reply 36):
my question the becomes, what is required for it to be in Normal mode?

The flight control system will be in Normal mode as long as the PFCs are receiving valid air data (from either the ADIRU or SAARU) and power (This includes availability of pitot heat), regardless of the state of the hydraulic systems.

Quoting mandala499 (Reply 33):
the secondary mode if I remember correctly latches on until you're on the ground.

Quoting BoeingGuy (Reply 34):
. If you cycle the Primary Flight Computer switch, it will select the highest available mode. If you are in Secondary, but the system is able to be in Normal and you cycle the PFC switch, it will go back to Normal.

This is correct. When you lose power with a dual flameout the system will drop to Secondary, but you can get it back to Normal once power is restored to provide pitot heating by cycling the PFC switch.

Of course, in the scenario of MH370, if the crew was unconscious at this point, there would be no-one to cycle the switch and so the system would remain in Secondary with no protections.

[Edited 2016-04-19 18:21:30]

WarrenPlatts · Fri Apr 22, 2016 10:49 am

Hold it. What you guys just said is interesting.

Quoting 7BOEING7 (Reply 32):
With the loss of both engines you drop into Secondary mode as indicated in Reply 30 by BoeingGuy and the only way to get back to Normal mode is to select the Primary Flight Controls disconnect switch to DISC then back to AUTO -- it won't go back to Normal mode just because there's electrical power.

Quoting BoeingGuy (Reply 34):
If you cycle the Primary Flight Computer switch, it will select the highest available mode. If you are in Secondary, but the system is able to be in Normal and you cycle the PFC switch, it will go back to Normal.

Quoting skyhawkmatthew (Reply 37):
This is correct. When you lose power with a dual flameout the system will drop to Secondary, but you can get it back to Normal once power is restored to provide pitot heating by cycling the PFC switch.

Cycling the PFC switch wouldn't cause the SDU to reboot, or would it?

One other question: What about the "shaking stick"? If the Primary Flight Control was in secondary mode, could the aircraft still tell if it was approaching a stall and cause the stick to shake?

mandala499 · Fri Apr 22, 2016 3:29 pm

Quoting WarrenPlatts (Reply 38):
Cycling the PFC switch wouldn't cause the SDU to reboot, or would it?

Satellite Data Unit? No. Not by a long shot. However, cycling the ADIRU might have an effect on the SDU.

BoeingGuy · Fri Apr 22, 2016 8:04 pm

Quoting mandala499 (Reply 39):
Quoting WarrenPlatts (Reply 38):
Cycling the PFC switch wouldn't cause the SDU to reboot, or would it?

Satellite Data Unit? No. Not by a long shot. However, cycling the ADIRU might have an effect on the SDU.

You can't cycle the ADIRU on the 777 in flight. The switch is inhibited in flight. You can turn the ADIRU switch to OFF, but it won't actually turn off.

The PFC Switch doesn't control anything but the PFC.

Quoting WarrenPlatts (Reply 38):
One other question: What about the "shaking stick"? If the Primary Flight Control was in secondary mode, could the aircraft still tell if it was approaching a stall and cause the stick to shake?

The Stick Shaker has nothing to do with the Fly By Wire system or Primary Flight Computer. It's controlled by the Warning Electronic Unit, which is not related to what PFC mode you are in. So in answer to your question, the Stickshaker would function regardless of whether you are in Normal, Secondary, or Direct Mode.

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