N139j From United States of America, joined Nov 1999, 380 posts, RR: 1 Posted (14 years 4 months 2 weeks 1 day 9 hours ago) and read 1697 times:
Can anyone help me? I was just flying MIA-ATL. Nearing FL480 (Cruise Alt), I noticed my IAS falling bellow 240. I checked autopilot to see if I had changed it from the 310knts I had orginally entered after passing FL010. Nothing worked, so I diverted to an airport I found in GPS, making an emergency landing. The aircraft, once off autopilot, was very erratic and out of control. I was able to calm it before landing, but I was off the RW due to the weird flying. Does anyone know what happened? All help is appreciated.
Nbirger From Canada, joined Mar 2001, 69 posts, RR: 0
Reply 1, posted (14 years 4 months 2 weeks 1 day 3 hours ago) and read 1659 times:
That seems like a very high flight level.
What aircraft were you flying?
You also have to remember that the higher you are flying the thinner the air is. So your KIAS (knots in air speed) will be far less than normal. When I fly a 747 at FL390 the KIAS is usually around 275-285 which is around mach .84 (the cruise speed for this a/c). The best tip I can give you is to switch over to mach hold instead of speed hold on the autopilot. For the HEAVIES use around mach .84. This is far more accurate at higher altitudes.