2011-10 · NASA ASRS report 976075
A Corporate aircraft Crew reported that after climbing to FL400 at ATC's request they found the temperature was ISA +11 and the aircraft lost airspeed so they started a descent while waiting for ATC clearance.
On our flight west bound Atlantic crossing we were level at FL360 about one hour and thirty-five minutes after departure. We asked for and were cleared to FL380; which we arrived at without issue. The OAT was cold; -52; and the aircraft was performing normal. ATC asked if we could go to FL400 for our ocean crossing. We accepted after reviewing our FMS for performance at that altitude and they indicated we could. As we started up we noticed a significant warming of the OAT above FL390 and when we arrived at FL400 the OAT was -46 with an ISA of +11. Due to aircraft weight and OAT; we where indicating a MACH of .67 to .68 with a rate of climb of +300 feet. After leveling off at FL400 with max power applied the aircraft would not accelerate. In fact; the indicate airspeed started to decelerate from 205 through 195 at a rather slow rate. After about ten minutes at max power and a slowly decreasing indicated airspeed I asked ATC for a lower altitude to which they responded; 'standby.' Within minutes for requesting a lower FL the indicated airspeed started a more rapid decrease with an increasing AOA; indicating back side of power curve; and with the OAT still indicating +11 I told ATC; 'we must descend now' to which we stated; 'roger; maintain FL400.' Shortly there after; I felt the wings start a slight buffet and a slight rolling of the wings. I checked the TCAS and seeing no aircraft within a 40NM range I instructed the pilot flying to descend now. I told ACT 'I AM descending and would level off at FL380.' After passing FL390 the OAT decreased again to -52 and ISA had cooled to +2. The aircraft again returned to stable flight and we continued. All weather reports we had received from our handler; Universal; indicated temperatures at altitude to be -52 or colder. However; we found out that at above FL390 the actual temperature was much warmer and we could not operate at those altitudes and our aircraft weight.
The cause of this altitude deviation was simple the aircraft was too heavy; the temperature was too hot and the engines had no additional thrust. Other factors included mission pressure from being fuel load restricted and late arriving passengers; warmer than normal temperatures at altitude; no accurate temperature forecast for FL400; and my not sticking to my gut feeling. One final factor: ATC seemingly would not clear us to a lower altitude. We both felt as though the situation was an emergency but didn't communicate that to ATC. In hind sight; if I were to allow myself to get in this situation again; perhaps we should declare an emergency.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.
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