Problems with 748

Device: iPhone 12
Operating system: iOS 17.3

Hey guys, last night I tried to fly a long haul while I was sleeping from ELLX-KLAX in the 748. I brought about 125,000 kg of fuel for the trip that was supposed to last just under 11 hours. When I reached cruise, it refused to level off. Even when I set the V/S to descend and the trim to -100, it wouldn’t level off. The speed was set at Mach .85, and the engines were throttled to 100% to keep up. Still, it didn’t gain any altitude, nor did it pass 200 knots ias.


Does anyone know what went wrong? I eventually decided to go to bed and hope the problem would work itself out… but it just ran out of fuel and crashed

3 Likes

Hello,

It seems as though you may have been cruising too high for the weight of the airplane.

Additionally, you may have climbed too quickly and lost airspeed in the climb. Therefore, the airplane was not able to speed back up at such a high altitude.

3 Likes

36000 in a heavy 748 is pretty much coffin corner, it would be very difficult or dare I say impossible to gain speed at that altitude. 280, 300, or 320 should work better. Also, 748 has a fuel problem so 11 hours probably only lasts like 8 even at the most efficient altitudes.

2 Likes

I accelerated to cruising speed immediately after passing FL100

Not sure I understand that… FL360 is very normal for a plane of that size

Well yeah at FL100 you have so much excess power, but you’re going to bleed airspeed on the climb and by FL300 you probably need to reduce climb to like 1000 FPM to not bleed too much. In the photo you’re at 172 knots, and your AoA looks horrific, you can’t regain speed reasonably from that altitude (you’d have to dip the nose to gain speed).

Not in IF it isn’t, older model.

1 Like

Exactly my problem. The AoA being at that level was completely against what I had set. I had it set to 2000 fpm, it retained that AoA no matter what I did. The only way I was able to dip the nose was manually.

Well, if you have 2000 FPM set and your airspeed starts bleeding, your AoA is going to further increase. That isn’t the autopilot’s fault, it just wants to maintain VS. Higher AoA = more drag and so your engines can’t keep up. This isn’t a problem exclusive to the 748 but some other planes have enough excess power to pull you out of it.

Side note: 2000 FPM in a heavy 748 at FL320+ is most likely a death sentence.

2 Likes

It stayed nosed up even when I reached cruising altitude

True but I just wanted to go to bed😂

1 Like

But it failed to recover because you can’t gain speed at that altitude with that AoA. Once again, pilot error, not anything wrong with the plane.

Valid honestly but you can just set like 500 FPM or something small like that and go to bed.

I love using the 748 but agree there are some small issues with long haul flights, its not a sleeper flight unfortunately. Its strange I can fly the 737-700 for up to 16 hours but the 748 as you say, 8 hours on a bad day and maybe 10 on a good day.

I have an unhealthy fear of screwing up the airspeed calculations and overspeeding

2 Likes

Well, I mostly meant above FL280. Getting to FL280 at 2000 FPM should be alright.

True. I’m still paranoid😂

Airspeed is food and water. If you want your flight to live, forget everything else until IAS (indicated airspeed) is safeguarded as the first priority.

So, forget about any expectations about VS, AoA, cruising altitude or anything else, as soon as your airspeed shows signs of deteriorating.

Airspeed Loss is your call to action.

You monitoring and safeguarding airspeed is what sets the limit:
1)on how fast you can climb (at any altitude and weight)
2)keeping the AoA from growing beyond normal
3)how high you can climb at your weight (max VS for maintaining airspeed will be continuously decreasing, until at some altitude it must be zero).

Fair enough. To be able to sleep though, again I would think about what you must do to preserve airspeed over any other consideration.

So, for example, if you still have your climb ahead of you, and you’re heavily loaded:

Before going to sleep, set a very shallow climb, for a very conservative altitude (again, airspeed preservation first and always!).

Later when you wake up, you can adjust your altitude up, with whatever VS works, when you are able to keep an eye on how airspeed holds up as you climb.

Airspeed is your health. It’s your warning indicator for when you are at the limits of the flight model and that you have to change something.

I didn’t see that till I turned up my screen brightness. But it certainly goes along with:
image

2 Likes

Like I said. Airspeed and AoA were stable until I reached cruising altitude. I’ve done this a million times and have never had trouble with this before. I adjusted trim, flaps, everything. The plane did not want to nose down unless I flew it by hand

Please read what I just wrote, maybe looking first at your airspeed:
image

My explanation above was about you allowing your speed to drop that far.

If you think it would help to upload a video of your flight, by all means do that. Your video will l likely show how your loss in speed progressed.

Right. Airspeed was stable. A/P was set to M .85. It went to that after AP pulled the nose up for no reason

This is not stable:
image

It’s quite likely

The AP was trying to maintain your settings, but you let the airspeed drop that 172kts.

I just test flew your aircfraft type with your fuel load (I guessed and pax and cargo), at the altitude you were at.

I let the speed drop to your low speed and waited to see what the AP did with my inputs. My AoA got to 14 degrees. Full throttle, altitude dropping, then speed continued to drop.

1 Like