Lear 35 first officer reports inability to maintain new cruise altitude of FL430 after consulting pe...

Narrative:

On a part 91 aircraft repositioning flight we had difficulty maintaining altitude with our lear jet 35. At FL400 and 16000 pounds with temperature at isa -10 and .77 mach; we determined via the bombardier performance charts that we would be able to climb to FL430. This was desirable because we were facing a minimum 65 KT headwind along the route and were trying to maximize fuel efficiency. At approximately FL42.8 and 15800 pounds; we noticed the temperature was at isa and that our climb rate had diminished to zero. Our speed was .65 mach and our aoa indicator showed -1 unit before the stick shaker. We had difficulty holding altitude and airspeed and the airplane felt mushy. Unable to hold altitude; the pilot not flying/second in command immediately requested lower from ATC. ATC asked how much lower; and the pilot not flying/second in command requested FL400. In response; ATC said; 'I see you are starting down now; maintain FL400.' by this time we were at FL42.5 and -1000 FPM. ATC asked if we would be able to maintain FL400 and the pilot not flying/second in command responded in the affirmative. The remainder of the flight was uneventful. In the future; we will understand that old airframes and engines don't always perform to the factory numbers and not to expect temperature to remain at a constant value below isa throughout climb.

Original NASA ASRS Text

Title: Lear 35 First Officer reports inability to maintain new cruise altitude of FL430 after consulting performance charts. Reporter suspects ISA temperature increase as well as older airframe and engines are to blame.

Narrative: On a Part 91 aircraft repositioning flight we had difficulty maintaining altitude with our Lear Jet 35. At FL400 and 16000 LBS with temperature at ISA -10 and .77 mach; we determined via the Bombardier performance charts that we would be able to climb to FL430. This was desirable because we were facing a minimum 65 KT headwind along the route and were trying to maximize fuel efficiency. At approximately FL42.8 and 15800 LBS; we noticed the temperature was at ISA and that our climb rate had diminished to zero. Our speed was .65 mach and our AOA indicator showed -1 unit before the stick shaker. We had difficulty holding altitude and airspeed and the airplane felt mushy. Unable to hold altitude; the pilot not flying/second in command immediately requested lower from ATC. ATC asked how much lower; and the pilot not flying/second in command requested FL400. In response; ATC said; 'I see you are starting down now; maintain FL400.' By this time we were at FL42.5 and -1000 FPM. ATC asked if we would be able to maintain FL400 and the pilot not flying/second in command responded in the affirmative. The remainder of the flight was uneventful. In the future; we will understand that old airframes and engines don't always perform to the factory numbers and not to expect temperature to remain at a constant value below ISA throughout climb.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	837808
Time
Date	200905
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZZ.ARTCC
State Reference	FO
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Make Model Name	Learjet 35
Operating Under FAR Part	Part 91
Flight Phase	Cruise Climb
Flight Plan	IFR
Person 1
Function	First Officer
Qualification	Flight Crew Multiengine Flight Crew Flight Instructor Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 90 Flight Crew Total 6500 Flight Crew Type 400
Events
Anomaly	Deviation - Altitude Undershoot Deviation - Procedural Clearance Deviation - Speed All Types Inflight Event / Encounter Weather / Turbulence

Lear 35 first officer reports inability to maintain new cruise altitude of FL430 after consulting performance charts. Reporter suspects isa temperature increase as well as older airframe and engines are to blame.