Stalling at Higher Altitudes During Climb

[mikerapp]

I have noticed that very often during climbout in some of the large jets (747, 777) when I am on autopilot the airplane will suddenly increase the nose-up angle until the airplane eventually stalls. This usually happens at about FL280. Of course, I can disengage the AP and recover, but does anyone know why this happens?
Thanks,
Foureyes

[damnlimey]

I've had a similar problem with the POSKY 767, trying to get to FL360 for a flight from SJC to HNL. I got no real answer from anyone last time I posted it. My assumption was (based on real life observations of long haul flights) that a fully loaded plane is too heavy to fly at those altitudes. In the real world the flight starts at a lower altitude (say FL310 or so) and as they burn off fuel during the flight they gradually get higher and higher.

This proceedure can't be done in FS2002, the altitude you declare in the flight plan is what you have to stick with (or else you get your knuckles rapped by ATC). I'm told, although I don't have 2004, that there's a "Request higher (or lower)" option in the ATC package.

I'd be interested to know from some of the real world big iron jockeys out there if my assumptions are correct.

Keith

[XMech]

You're right. Since the air is thinner as you go higher, it's better because you have less drag, but you also get less lift and less thrust. But you still are going faster burning less fuel. I won't get into all the gorey details, but an extreme example is the SR-71. In it's high altitude, high-speed flight, it's going something like 2,000 miles an hour at 90,000 feet, but the airspeed indicator is reading 250 knots.

Another example I've heard of was of a 747 which was carrying an extra engine slung under the wing. (This is the only way to carry another engine without splitting the fan from the hot-section, with fairings installed on the engine, and attached inboard of the other under the wing.) During climbout, (pretty high already) they started experiencing a vibration and thinking something was amiss with the spare engine installation, they returned to their originating airport, making for alot of happy passengers. (I'll bet! ) Turns out nothing was wrong, except they didn't add the weight and drag of that engine and they were experiencing a pre-stall buffet!

[heyer]

The problem is that the standard FS autopilot is not capable of VNAV. In real aircraft when reaching FLAP UP and SLATS IN speeds the A/P is engaged and will keep 250kts up to FL100, there the nose will be lowered in order to accelerate to the climb speed. This speed will be calculated by the FMS or can be entered in the climb page. Normal climb speed for jets is around 310-330kts depending of weight and/or type of aircraft.

There are payware addons available which simulate the VNAV function quite good. I personally like the "Dreamfleet" B737-400 most.

[JerryH]

Foureyes,

Heyer gave you the answer when he mentioned climb speed. Maintaining the correct speed during climb is mandatory. As the aircraft reaches higher altitudes, minimum and maximum airspeeds get closer together as the flight envelope gets narrower. Near the ceiling altitude, min and max speeds will be very close, so there is little margin for error in controlling airspeed.

Here's how I perform the climb for the default B747 at a maximum takeoff weight of 853,000 lb. Before takeoff, set Mach to 0.90 (or some speed above what is planned). Pre-set altitude to the planned cruise altitude (or higher if I'm going for the ceiling). Set the climb rate to about 3500 fpm. After takeoff, when gear and flaps are up, engage the A/P. Continuously monitor airspeed! It should be gradually increasing. If not, lower the climb rate.

When the climb is stable, arm the Auto Throttle and click on Mach. This will gradually reduce the thrust from Takeoff power to Max Continuous power. Monitor and control airspeed by adjusting the climb rate. Stay below 250 KIAS until reaching 10,000 ft.

Above 10,000 ft, lower the nose (lower the climb rate) and accelerate to 340 KIAS. Try to reach 340 KIAS by the time you get to about 20,000 ft. Maintain 340 KIAS (by raising the nose a little) until reaching Mach 0.84. Maintain Mach 0.84 by adjusting the climb rate until the rate gets very low (500 to 1000 fpm). At that point, you're at or near the ceiling altitude for that gross weight condition. (Note that when climbing at constant Mach number, IAS is slowly dropping. This is correct, but ignore IAS and hold Mach constant.)

When I follow the above procedure, the B747 gets to 30,300 ft at a climb rate of 1,000 fpm. It will go higher if the climb rate is further reduced, but it takes too long to climb when you're at 500 fpm and dinner is on the table getting cold. About the only ways to greatly increase the ceiling is to reduce the fuel load or cheat and climb all the way at takeoff power (no A/T).

Refer to the FS2002 Aircraft Handbook for the correct climb speeds. See page 48 for the B747 and page 57 for the B777.

Please let us know if any of this helps.

Regards,
JerryH