CL600 flight crew has an altitude deviation while in cruise.

Attributes
ACN	702133
Time
Date	200607
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : eisn.airport
State Reference	FO
Altitude	msl single value : 38000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : eisn.artcc
Operator	general aviation : corporate
Make Model Name	Challenger CL600
Operating Under FAR Part	Part 91
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : corporate
Function	flight crew : captain oversight : pic
Qualification	pilot : multi engine pilot : atp pilot : commercial
Experience	flight time last 90 days : 120 flight time total : 16500 flight time type : 3600
ASRS Report	702133
Person 2
Affiliation	company : corporate
Function	flight crew : first officer
Qualification	pilot : commercial pilot : atp
Experience	flight time last 90 days : 99 flight time total : 11940 flight time type : 297
ASRS Report	702140
Events
Anomaly	altitude deviation : excursion from assigned altitude inflight encounter other non adherence : clearance
Independent Detector	other flight crewa other flight crewb
Resolutory Action	controller : issued advisory controller : issued new clearance flight crew : exited adverse environment
Consequence	faa : reviewed incident with flight crew
Supplementary
Problem Areas	Flight Crew Human Performance ATC Human Performance Aircraft
Primary Problem	Flight Crew Human Performance

Narrative:

We were cleared to FL380 and cleared direct to nibog intersection. At leveloff the temperature was isa +2 degrees and we had isa +4 degrees programmed into the FMS. For our weight; altitude; and temperature there was no warning from the FMS stating 'unable cruise altitude.' at the top of climb the mach fell to .71 at isa +2 degrees. The FMS was programmed for each leg with both temperature and wind for each individual leg and the FMS did not give a warning of 'unable cruise altitude' for the whole flight at FL380. Within a very short period of time the temperature went to isa +8 degrees and mach fell to .66. We twice asked scottish control for lower (FL370) and did not get a clearance for lower or even a response. Scottish control then handed us off to shannon; the mach was still dropping and the aircraft started a very light buffet. We told shannon unable to maintain FL380 and needed immediate descent clearance to FL370. With aircraft still in a light buffet we told shannon we were unable to maintain FL380 and descending to FL370. At about FL374 shannon cleared us to FL370 and advised us of an altitude bust. We told the controller that we were unable to maintain altitude due to rapid temperature rise. Fortunately there were 'no' other aircraft displayed on TCAS. The temperature rise was from isa +2 degrees to isa +8 degrees. We were reclred for oceanic crossing at FL370 and the remainder of the flight was uneventful. To avoid this type of situation in the future ATC should accept the filed altitude and clear aircraft as such not try to get higher altitude out of crew. Supplemental information from acn 702140: we had made our decision to accept FL380 based on the forecast temperature; our weight at cruise and the idea of opening up lower altitudes for other aircraft. The performance numbers were close but we felt there was enough margin and the FMS to xchk the decision. In retrospect; if I cannot fly the altitude at isa +10 degrees I will not go there with this aircraft. Second; we should have declared an emergency after our second request for a lower altitude got no relief because shortly after that we had no choice but descend. For who knows what reason we seem to be reluctant to do that unless we are on fire or out of gas.

Original NASA ASRS Text

Title: CL600 FLT CREW HAS AN ALTDEV WHILE IN CRUISE.

Narrative: WE WERE CLRED TO FL380 AND CLRED DIRECT TO NIBOG INTXN. AT LEVELOFF THE TEMP WAS ISA +2 DEGS AND WE HAD ISA +4 DEGS PROGRAMMED INTO THE FMS. FOR OUR WT; ALT; AND TEMP THERE WAS NO WARNING FROM THE FMS STATING 'UNABLE CRUISE ALT.' AT THE TOP OF CLB THE MACH FELL TO .71 AT ISA +2 DEGS. THE FMS WAS PROGRAMMED FOR EACH LEG WITH BOTH TEMP AND WIND FOR EACH INDIVIDUAL LEG AND THE FMS DID NOT GIVE A WARNING OF 'UNABLE CRUISE ALT' FOR THE WHOLE FLT AT FL380. WITHIN A VERY SHORT PERIOD OF TIME THE TEMP WENT TO ISA +8 DEGS AND MACH FELL TO .66. WE TWICE ASKED SCOTTISH CTL FOR LOWER (FL370) AND DID NOT GET A CLRNC FOR LOWER OR EVEN A RESPONSE. SCOTTISH CTL THEN HANDED US OFF TO SHANNON; THE MACH WAS STILL DROPPING AND THE ACFT STARTED A VERY LIGHT BUFFET. WE TOLD SHANNON UNABLE TO MAINTAIN FL380 AND NEEDED IMMEDIATE DSCNT CLRNC TO FL370. WITH ACFT STILL IN A LIGHT BUFFET WE TOLD SHANNON WE WERE UNABLE TO MAINTAIN FL380 AND DSNDING TO FL370. AT ABOUT FL374 SHANNON CLRED US TO FL370 AND ADVISED US OF AN ALT BUST. WE TOLD THE CTLR THAT WE WERE UNABLE TO MAINTAIN ALT DUE TO RAPID TEMP RISE. FORTUNATELY THERE WERE 'NO' OTHER ACFT DISPLAYED ON TCAS. THE TEMP RISE WAS FROM ISA +2 DEGS TO ISA +8 DEGS. WE WERE RECLRED FOR OCEANIC XING AT FL370 AND THE REMAINDER OF THE FLT WAS UNEVENTFUL. TO AVOID THIS TYPE OF SITUATION IN THE FUTURE ATC SHOULD ACCEPT THE FILED ALT AND CLR ACFT AS SUCH NOT TRY TO GET HIGHER ALT OUT OF CREW. SUPPLEMENTAL INFO FROM ACN 702140: WE HAD MADE OUR DECISION TO ACCEPT FL380 BASED ON THE FORECAST TEMP; OUR WT AT CRUISE AND THE IDEA OF OPENING UP LOWER ALTS FOR OTHER ACFT. THE PERFORMANCE NUMBERS WERE CLOSE BUT WE FELT THERE WAS ENOUGH MARGIN AND THE FMS TO XCHK THE DECISION. IN RETROSPECT; IF I CANNOT FLY THE ALT AT ISA +10 DEGS I WILL NOT GO THERE WITH THIS ACFT. SECOND; WE SHOULD HAVE DECLARED AN EMER AFTER OUR SECOND REQUEST FOR A LOWER ALT GOT NO RELIEF BECAUSE SHORTLY AFTER THAT WE HAD NO CHOICE BUT DSND. FOR WHO KNOWS WHAT REASON WE SEEM TO BE RELUCTANT TO DO THAT UNLESS WE ARE ON FIRE OR OUT OF GAS.

Data retrieved from NASA's ASRS site as of January 2009 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.