Fuel Exhaustion Flameout Realism

[whitav8 98]

Has the realism of the sequence of events and hands off flying after a Fuel Exhaustion flameout on the PMDG 777 even been discussed? Here is what I've seen:

 

 I noticed  that if I simply let the fuel run out and don’t touch a thing (previously I was setting a VertSpd descent rate at engine fail), first the engines fail with 300 lbs of fuel left, the autopilot holds altitude (I was at over 30K feet at Mach = .85) and lets the speed bleed off until it gets to 170 knots or so and then the stall warning goes off and the autopilot disconnects. Then the nose lowers under FlyByWire control and the aircraft descends at about 2000 fpm so the airspeed stays at 180 knots or so all the way to the surface. Either the system is running on batteries, the APU (which would autostart with a little fuel in its tank), or the RamAirTurbine prop comes out – just to keep the FBW alive. 

 

[scandinavian13 5,192]

 

 

 I noticed  that if I simply let the fuel run out and don’t touch a thing (previously I was setting a VertSpd descent rate at engine fail), first the engines fail with 300 lbs of fuel left

 

There exists a concept in aviation called unusable fuel (yes, I know the linked article is for usable fuel, but unusable fuel is also discussed).  Ever try to scoop ice cream out of a carton?  You'll notice that you can't scoop all of it out and some of it goes to waste.  Same concept.

 

 

 

the autopilot holds altitude (I was at over 30K feet at Mach = .85) and lets the speed bleed off until it gets to 170 knots or so and then the stall warning goes off and the autopilot disconnects

 

What did your FMA say?  The autopilot is a tool to assist you in flying the plane.  It isn't some all-knowing wonder that will save you in an emergency.  It's going to continue trying to do whatever you last told it.  The plane knows the engines failed.  The autopilot couldn't care less.  It does what you've instructed it to do.

 

 

 

Then the nose lowers under FlyByWire control and the aircraft descends at about 2000 fpm so the airspeed stays at 180 knots or so all the way to the surface.

 

Okay.  What's your point here?  Without thrust to maintain speed/altitude, the plane has to descend to maintain enough forward speed to continue flying.  This is true for every aircraft out there.

 

 

 

Either the system is running on batteries, the APU (which would autostart with a little fuel in its tank), or the RamAirTurbine prop comes out – just to keep the FBW alive. 

 

The RAT will pop out automatically if it's necessary.  Similar to the engines, the APU is dependent on fuel.  If the engines flamed out because of a lack of fuel (refer to above: usable fuel), then the APU isn't going to fare any better.

[benrussell 20]

@kyle:

 

 

;)

[scandinavian13 5,192]

 

 

@kyle:

 

My life will never be the same  :o

haha

[77west 139]

 

 

@kyle:

Now THAT - Is a clever idea!

[indyrocks 2]

It's not the same without Carly Simon singing "Anticipation".

 

An example of the late 70's ad campaign (starring a very young Corey Feldman, with the late Casey Kasem as the announcer) can be found here. Of course, plastic bottles were not in widespread use then.

[whitav8 98]

 Not sure how this thread got hijacked, but I am simply trying to answer some questions about Malaysian Flight 370 using the wonderful PMDG 777 as a tool to try various scenarios. I simply wanted to know from any real 777 pilots or others in the know about whether what I have observed with a dual engine fuel exhaustion flameout  and no further pilot intervention (but with either FMS LNAV or Autopilot Track Select mode engaged) is accurate to the real aircraft. I have seen that PMDG did a terrific job with most issues and was seeking to find more about this event.

[willywonka 36]

 

 

about whether what I have observed with a dual engine fuel exhaustion flameout  and no further pilot intervention... is accurate to the real aircraft.

Yes it is. Why wouldn't it be?

[whitav8 98]

This is a quote from pprune.org from a 777 pilot. As you will see, this result is different from what happened with the PMDG 777

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As a 777 pilot I, like many others, have wondered how the 777 would perform in the scenario where the pilots were incapacitated and the aircraft ran out of fuel. I had my ideas but there is nothing like seeing it for "real" so we tried this in a 777-2 full motion zero flight time approved simulator. 

We used a zero fuel weight of 175 tonnes. We let it run out of fuel at FL 250 in track hold and alt capture. However it would not make any difference what mode it was in as everything would drop out. In real life one engine uses fractionally more fuel per hour than the other and there is typically a difference between main tanks of a few hundred kilos, so we had a 300 kg difference between the contents of the left and right tank.

When the first engine failed TAC (Thrust asymmetry compensation) automatically applied rudder. The speed reduced from 320 knots indicated to 245 knots indicated. It was able to maintain 245 knots and FL250. When the second engine failed the rudder trim applied by TAC was taken out and the trim went to zero. The autopilot dropped out and the flight controls reverted to direct mode. The speed initially came back to 230 knots but then the nose started to lower. The nose continued to lower and the rate of descent increased to 4,000 feet per minute, the nose kept lowering and the descent rate increased to 7,500 feet per minute with a bank angle that increased to 25 degrees. The speed at this point had increased to 340 knots indicated, above VMO but there was no horn as it was on limited electrics. About this point the RAT (Ram air turbine) chipped in and the CDUs and copilot's PFD (Primary flight display) came alive. The flight controls stayed in direct mode.The eicas screen was full of messages like pitot heat, flight controls, APU fault (The APU had tried to autostart due double engine failure but failed due no fuel to start it) low fuel pressure etc. 

Then with a max descent rate of almost 8,000 feet per minute the nose started to slowly rise and keep rising. We had dropped to about FL170 but the nose slowly rose up to 6 degrees pitch up and we started climbing at about 3000 feet per minute and the bank angle reduced to only 5 degrees. It climbed back up to FL210 at which point the speed had come back to 220 knots and then the nose dropped down again and we were soon back to descending at 8000 feet per minute. So basically a series of phugoid oscillations with bank angle between 5 and 25 degrees and pitch attitude between about 9 degrees nose down and 6 degrees pitch up. It was losing about 8000 feet and then gaining about 3 or 4000 feet with airspeed fluctuating between 220 and 340 knots.

We didn't watch it all the way down due time constraints and stopped the experiment at 10,000 feet but it was consistent all the way down. Having watched it I can say with certainty that if the pilots were incapacitated and it ran out of fuel there is no way it could have landed on the water with anything like a survivable impact. Just passing on the info.

[FLEX1978 373]

Yes it is. Why wouldn't it be?

 

No it wouldn't.  A Level D simulator would not be entirely accurate let alone a desktop sim like PMDG.   As far as the system logic and behavior the PMDG sim would do a good job of replicating the real thing, as far as the actual flight behavior you would see a very basic representation of how the real aircraft would perform.   

[willywonka 36]

 

 

As far as the system logic and behavior the PMDG sim would do a good job of replicating the real thing, as far as the actual flight behavior you would see a very basic representation of how the real aircraft would perform.

Yea, but all we're talking about here is what happens at flame out. There's no magic or hidden mode in the real plane that's not in PMDG's. The plane will just try to continue to do what it was last told to do. Nothing more, except for some redundancies that can be expected to kick in.

 

The plane will just drift down and undergo a series of oscillations. It'll pick up airspeed, climb until it can't and dive again, assuming there's no pilot. It's the same whether it's a Cessna, or a 747, or a 777. Not sure what else can be expected other than that.

[FLEX1978 373]

Yea, but all we're talking about here is what happens at flame out. There's no magic or hidden mode in the real plane that's not in PMDG's. The plane will just try to continue to do what it was last told to do. Nothing more, except for some redundancies that can be expected to kick in.

 

The plane will just drift down and undergo a series of oscillations. It'll pick up airspeed, climb until it can't and dive again, assuming there's no pilot. It's the same whether it's a Cessna, or a 747, or a 777. Not sure what else can be expected other than that.

 

 

Well in that case I will say that Mario Kart is an accurate driving simulator :) Read post 7, the op is asking about realistic sim behavior  in reference to MH370 and the flame out scenario.  The PMDG sim will only give you a basic representation of the how the real aircraft will behave, nothing more nothing less.  

[kevinh 409]

Currently the PMDG 777 has no chance of doing this realistically as the FBW will auto trim with speed changes. The 777 FFS degraded to Direct Law, from what the OP said that didn't happen with the PMDG sim so that would create another difference. So the PMDG sim will be much more stable. After SP1 it might respond more like full flight sim. Even the FFS might not simulate the logic of this 100% accurately but it would be a good guide.

[willywonka 36]

 

 

the op is asking about realistic sim behavior...The PMDG sim will only give you a basic representation of the how the real aircraft will behave, nothing more nothing less.

Fair enough! But we would have to wait until SP1 is released to see how the new, more accurate FBW system reacts.