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I was doing a flight from YMML to WMKK in a 777 200er on expert server. And I reached my Cruise altitude of 38,000 ft where I stayed for 2hr before a random amount of proposing/ pitch up followed by pitch down occurred causing a crash. Can someone help me understand why this happened?
Welcome to the community!
Could you share a replay of the moment and prior to that so that we can help?
how do I share video
Try screen recording then uploading to YouTube (can be unlisted) or try https://sharemyinfiniteflight.com/
what was your aircraft load? did you climb directly to cruising altitude after takeoff? the oscillating pitch up/down movement is usually linked to being too heavy for the altitude you’re at.
He said he was at cruise for 2 hours before this so I don’t think the climb is the issue.
While yes, usually this is due to being too heavy, the 772 has a LOT of excess power so I wanted to test something. When we see people flying fine for an hour or two and then stalling, it’s usually because they’re flying on the bleeding edge of the performance envelope. From there, wind anomalies (for example, wind going from 50 knot headwind to 0 instantly) occur from which the plane doesn’t have enough power to recover from.
I ran a Simbrief of this and it led to a load of about 55%. Took the 772 to FL380, stabilized it, and then turned on a 100 knot direct tailwind. This results in the KIAS dropping to below 200 (basically, if the plane can recover from this then there’s no way weight is the cause of the issue). It recovered without a hitch. In fact, it seems like the 772 can recover from a 100 knot instant tailwind up to 70% load. This essentially means there’s no way weight should’ve been a factor here unless you were for some reason well above 70%. Winds in the area are around 10-50 knots, so even with a complete wind glitch/reversal your plane should’ve still been able to recover.
Knowing this, I’m wondering if something else could be at play. As others mentioned, a replay would be helpful.
https://youtu.be/jks6feVBofA?si=2izoFHq4PdINyM8t That’s the link for the screen record
The B77W is very well know for it’s initial low cruising altitude. On heavy and long flights this could sometimes even be at FL280/FL300 (initial cruising altitude) . Even with being 2 hours cruising beforehand FL380 is still WAY to high.
A sudden change in wind speed could be the final blow for the aircraft and it wouldn’t keep up the speed till then. But the primary reason is a to high cruising altitude relative to the weight your Aircraft has. Do you have any info about aircraft weight, cargo? Fuel? Pax?
aw, can’t see the winds in replays unfortunately. But I think @AndrewWu 's guess is probably right.
@Ned_Scrim let me guess… your THR% was above 90% or around 95% when you were at FL380? also… did you step climb at all?
Simbreif said my initial step climb should be 38,000 before a final of i think 41,000 but i guess i probably shoulda stopped at 33 just to be safe. but i was flying at mach .86 which was around mid 80% i believe
Simbrief airframes are not the same as IF weights etc, you usually need to edit airframe weights
I was able to reproduce your pitch oscillations followed by a stall and dive.
Cruising relatively heavy at FL380 and stable at cruise speed, my extra AoA was like yours (to generate necessary lift when heavy and high).
I noticed at the start of your pitch oscillations your IAS had already dropped substantially even with AP engaged (presumably), consistent with a change in wind.
I found when my IAS droppped as low as yours, when I changed the wind, it was near the entry point of oscillations and a “speed trap” where drag starts to exceed available thrust.
I did have to put in a bit of temp increase to hurry it up “over the edge.” The IAS gradually dropped leading to the stall.
But the instability from high and heavy (exaggerated AoA in level flight), combined with a shock from change in weather conditions, was quite reproducible.
What load was this at? I didn’t have issues until like 80% and 100 kt change in winds. Wasn’t even near full throttle to maintain level flight.
I was well heavier than that the first time.
But I also tried 75% load starting at 15C.
Putting in 100kt tail wind (when at a stable cruise speed), oscillations already started I believe, without adjusting temp. Increasing temp 10 to 15C, and I couldn’t maintain speed at full throttle.
There seems to be a tipping point somewhere around the above in the exact mix of altitude, load, speed, wind and temp (density altitude).
edit:
btw, Welcome to the community @Ned_Scrim!
I went back and tried again. The trigger I found for relatively high and heavy when I tried to reproduce your conditions, and assuming an increasing tail wind, was elevated temperature (@AndrewWu).
Setting a sea level temp of 35C is hot but not at all unusual in the summer. And it definitely made full throttle unable to sustain IAS (indicated airspeed) - I kept losing forward speed.
Your route was Melbourne to Kuala Lumpur I believe. It’s hot summer in Australia and you said you had about two hours of flight time before the problem occurred.
Flight time from Melbourne to Alice Springs I think is just under 3 hours. Though you would presumably have been flying somewhat south of the route to Alice Springs, after a couple hours in that general direction you’d likely be above the heat of outback Australia in the summertime.
Although cold at FL380 altitude, the density altitude (effective altitude) from the hotter sea level temperature makes your aircraft feel like it’s flying substantially higher than FL380 (it’s performing like it’s some thousands of feet higher).
This is a real-life effect that reduces the maximum safe load you can carry at any actual altitude.
Forgot about temperature tbh. Will incorporate that into future tests.
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