Fuel predictions and VNAV operation above FMC MAX ALT

[Avi]

Hi,

I'm reading a (old?) Flight Crew Training Manual I found on the web and it says that FMC fuel predictions are not available above FMC MAX ALT and are not displayed on the CDU. It also says VNAV is not available above FMC MAX ALT.

In PSX both exist. Should they?

Cheers,

[Hardy Heinlin]

Hi Avi,

whether VNAV remains engaged above MAX ALT, this question would require a test in the big sims: Disengage the A/P but keep the F/D engaged in VNAV PTH or VNAV ALT or VNAV SPD, and climb manually a few hundred feet above MAX ALT. Maybe there'll be a yellow line across the VNAV word on the FMA?

I don't know the answer to the other question either. Maybe the predictions will be dashed then.


Regards,

|-|ardy

[Avi]

When I did my test, while above MAX ALT there was a turn. During the turn the speed dropped below minimum maneuvering speed and at some point I did get VNAV...

[Hardy Heinlin]

:-) I wasn't thinking of the low speed effect; that would also affect the other pitch modes.

For a plain FMC software test, I guess one would have to enter a fictitious, higher GW on the PERF INIT page while the true GW is a lot lower, and then fly, say, 2000 feet above that entered MAX ALT at Mach 0.9 and play with the VNAV switch on the MCP. Perhaps it'll be impossible to change from V/S, for example, to VNAV; but once VNAV is engaged, it probably remains engaged.


|-|ardy

[double-alpha]

Hello

My 787 FCTM says:

FMC fuel predictions are not available above the FMC maximum altitude and are not displayed on the CDU.
VNAV is not available above FMC maximum altitude.

I don't know for the 747

[simonijs]

I am just guessing here, because I don't know for sure. But...:

If you are flying at OPT ALT for a long time, TAS will gradually decrease (say: from 495 to 487 kts, and so would the Mach-number). If you then climb to the next higher available optimum level, density will decrease and hence TAS will increase back to - say - 495 kts. Since temperature usually decreases with increasing altitude, now Mach will increase as well to a number well above that at the previous level (say: from M0,85 to M0,86). With engines being powerful enough, you can only climb "by so much" up to the point where an increase in Mach results in approaching the critical Mach-number. If the aircraft was to climb beyond the MAX ALT*, with further decreasing densities and temperatures, the Mach-number could thus become very high, perhaps even exceeding the critical Mach-number where drag will rise enormously. Prediction of Fuel Flow becomes impossible. At the same time, Thrust available may become insufficient to meet Thrust required (= Drag). Then speed can no longer be maintained, nor the Path. But again...: a wild guess.

* The FCOM contains tables for MAX Operating ALT. One number is for "Buffet Limit Pressure Altitude, based on 1,3G bank maneuver capability at 8,5 % MAC", the other for "Maximum Climb Thrust Limited Pressure Altitude and 100 ft residual Rate of Climb". The Buffet Limit PA is always the lower number. I don't know which altitude is shown on the VNAV CRZ page, on the real aircraft or in PSX.

Regards,
Simon (in for some coffee now).

[Britjet]

The aircraft can actually climb higher than the predicted Max Alt. There is a margin of about a thousand feet or so, as I recall.
Of course on all jets there will be a trade-off between the capability of the engines, and the capability of the wing. With the huge excess thrust typically found these days it's usually the wing that is the issue - the infamous coffin corner. In earlier times it was much more serious because once you got there you didn't have excess thrust to get you out of a slow situation.
I've never exceeded Max Alt for real but if you get close, a large bank does run the risk of a low speed warning or even a stick shake.
My gut feeling is that you won't get the predictions on the FMC, they will just be blank, and the Autopilot will be "yellow-lined"
Peter

[Jeroen Hoppenbrouwers]

Relevant quote from Richard de Crespigny, QF32:

I remember once on the way to London, we were in the 747-400 at 35,000 ft, very close to our maximum altitude. Meanwhile, a 747-400 from another airline came from behind and 4000 ft above us. Our instruments showed we were cruising with a speed margin of plus/minus 5 knots. We would get supersonic buffet if we would speed up 5 knots, and we might stall in a turn if we slowed down 5 knots. We didn't know the performance of the 747-400 behind us, but we figured (since it also was flying from Asia to Europe) they were also heavy and so shouldn't be that high. They must have had no speed margin to play with. We then passed a waypoint where the route turned through a 40-degree angle, an unusually large change in direction. The G force came on as our aircraft banked into the turn, and our speed tapes showed the previous 5-knots margin was now pinched down to almost zero. The thrust increased to the maximum cruise limits and, bit by bit, we nudged our way through that turn. We were fine but the same was not true for the trailing 747-400 -- they hit and then penetrated Coffin Corner. The speed tapes must have already shown little margin when they were flying straight. But now, in the turn, their speed margin reduced to zero and the engine thrust at the higher altitude was insufficient to counter the increased drag. With stalled wings and insufficient thrust they only had one place to go -- down!

"PAN PAN PAN -- [callsign] in an emergency descent to 35,000 ft." They plummeted down through those 4000 feet in about 30 seconds.

It was a terrible example of airmanship. The engineer in a 747 Classic would have warned the pilots not to climb so high, but the engineer had been replaced by computers and the pilots did not understand their jet's performance. They probably understood the yellow no-go speed zones on the speed tape, but they didn't know the autopilot would mindlessly bank the aircraft into a 20-degree banked turn at high altitude. They could have negotiated the turn more safely if they knew they could limit the bank angle, but they were clueless.

Also compare the more recent mishap where a captain forgot to fly at higher airspeed when he had to hold higher than expected and stalled out as well.


Hoppie

[Hardy Heinlin]

Good evening,

has anyone ever tried the following test on a real Honeywell FMC during preflight?

• On the CRZ page enter a CRZ ALT higher than the MAX ALT. Do not execute.
• The 3rd line ("W/MOD" line) normally shows the predicted ETA/FUEL at the destination -- is it blank?
• On the RTE DATA pages, is any waypoint fuel blank that lies after the first MAX ALT exceedance?
• Push ERASE.

Of course, when changing from ACT to MOD, the FMC will blank all fuel predictions for some seconds. The question is: Will it ever redisplay any fuel data for waypoints that lie after the MAX ALT exceedance point?

Any thoughts?


Regards,

|-|ardy

[Mariano]

I will try it today and report.

Best regards,

Mariano

[Mariano]

Hardy,

Please see below.

https://1drv.ms/v/s!AsRrLL6HC4nag71iMFDAhDBVUb3LXg

Best regards,

Mariano

[Hardy Heinlin]

Thank you, Mariano.

So it doesn't even display the fuel data for the climb waypoints (except for the first one).

Was this the newer, faster FMC box that would display the MOD data within 3 seconds? I learned it may take half a minute on the older boxes until the MOD data appear.


Regards,

|-|ardy

[Mariano]

Hardy,

It is a PIP, so not as fast as Pegasus. I have one more sector to do today.

I will try it again and wait a bit longer, which makes a lot of sense.

Best regards,

Mariano

[Toga]

🦸‍♂️

[Mariano]

Hardy,

I checked again and recorded a two-minute long video while I waited for modification to perhaps display fuel figures in route waypoints, which it did not. Nevertheless, it did display fuel for the VECTORS portion, which was an unexpected result.

I could upload the 55 MB video, but there is no use to doing so. I took a screenshot of the MCDU displaying the VECTORS fuel, just in case You want to see it (link below).

https://1drv.ms/u/s!AsRrLL6HC4nag71jKRbM1VUk2m49Ww

Best regards,

Mariano

[Hardy Heinlin]

Thank you, Mariano.

I think it displays the fuel for the next waypoints that are less than 20 or so miles from the aircraft.

This fuel blanking feature is now implemented in PSX, but it blanks it for all waypoints as I don't know the logic in detail, obviously. Also, it won't blank the entire "W/MOD" line but just the fuel part of it. How this would look like on the real 744 FMC in detail is just speculation; I don't want to fiddle about so much with hypothetical details :-)


Cheers,

|-|ardy

[emerydc8]

Hey Mariano,
What's with the Casio Data Bank watch? I thought I was the only  holdout who still wears one of those. They're getting expensive to find new.

Jon

[Mariano]

 

I thought I was the last hold out!

Had it for well over a decade; they are indestructible.

They, together with cockroaches, will be the only two things that will remain intact inhabiting Earth after a nuclear holocaust.

I received my first one in the late eighties. Had one since. Let's just say I'm not too big on change... ;-)

Regards,

Mariano

[Hardy Heinlin]

The fuel display blanking feature is now available in PSX 10.115:

http://aerowinx.com/board/index.php?topic=4191.0


|-|ardy

[emerydc8]

Hi Hardy,

To add to the big picture, I tried a few experiments on my flight tonight from LAX. First, while cruising at FL350, our max altitude was FL388. I re-cruised VNAV for FL400 to see whether it would give fuel calculations. The answer is no. In fact, it won't even give data at FL390 (I entered that at 01:05 in the video). On another attempt I did leave the higher altitude in there for several minutes to see if it was just the slow processing, but I don't think it is. In fact, when I switch back to FL350 you can see how quickly it re-calculates the fuel data.

https://youtu.be/C5pxNrNYBII

Further, and since I didn't feel like entering the ranks of Test Pilot tonight by climbing above the max VNAV altitude, I just decided to add an extra 100,000 pounds of fuel to the FMC where our actual altitude (FL350) would be above the new max VNAV altitude of FL346. I thought it was interesting that the FMA goes from VNAV PTH to V/S. I didn't leave it this way for long because I don't really want to get a call from the FOQA people and have to explain exactly what I was doing.

https://youtu.be/Ky8yqKbV4m8

Also, this experiment of adding extra fuel only worked when VNAV was already engaged.  After I tried this experiment the first time, I selected ALT HOLD to maintain altitude. When I restored the fuel to the original value, then added 100,000 pounds and tried to engage VNAV, it stayed in ALT HOLD. It is notable that I had the VNAV light (in the switch) and the ALT HOLD light (in the switch) illuminate simultaneously at this point. I'm sure this really confused the FMC. I guess it means that VNAV will not even engage when it's above the max VNAV altitude. The VNAV light in the switch illuminates but the FMA stays in ALT HOLD. The only way I could get the original experiment to duplicate itself was to restore the fuel, go back to VNAV and then add the fuel once it's in VNAV. Then it would show V/S once I input the new fuel load.

Also, when I input the new fuel load, the A/T SPD blanked momentarily and re-displayed SPD, while at the same time, for a second, the pitch mode just indicated VNAV (00:15), then it went to V/S. The V/S button internal light also illuminated when V/S was displayed. I got an EICAS FMC warning as well.

Sorry about the camera bouncing around. I was trying to make sure the airplane continued to fly straight-and-level while all this was going on.

Jon