Narrative:

First officer inadvertently entered wrong outside air temperature into flight management computer on takeoff page, thereby selecting less thrust than required for conditions. He typed in and then entered '50' instead of '50F.' when a # is entered, it is assumed by the computer to be degrees celsius unless followed by an 'F.' so we had an N1 engine speed setting based on 50 degrees C OAT vs 50 degrees F. Fortunately, I noticed the lower N1 setting and pushed the thrust lever farther forward, but it took us a while to discover where the problem was and how the error had occurred. I recommend that the manufacture in grand rapids, mi add a software safety valve or OAT 'reasonableness test' that would help catch such errors. One way to do this would be to feed actual OAT or static air temperature (sat) from the tat/sat/TAS indicator to the FMS and have the FMS compare manually entered OAT valves vs measured actual OAT and alert the crew via an FMS text alert prompt in the FMS CDU scratchpad to the apparent discrepancy. I also recommend that the FMS software be changed to require that a 'C' or an 'F' follow the temperature entry or else the computer would advise 'invalid entry' for only a # entry attempt on the takeoff page.

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Original NASA ASRS Text

Title: AN UNDER-POWERED TKOF WAS STARTED AS FO PLACED IMPROPER SENSING FACTORS IN FMC FOR ENGINE POWER PARAMETERS.

Narrative: F/O INADVERTENTLY ENTERED WRONG OUTSIDE AIR TEMP INTO FLT MGMNT COMPUTER ON TKOF PAGE, THEREBY SELECTING LESS THRUST THAN REQUIRED FOR CONDITIONS. HE TYPED IN AND THEN ENTERED '50' INSTEAD OF '50F.' WHEN A # IS ENTERED, IT IS ASSUMED BY THE COMPUTER TO BE DEGS CELSIUS UNLESS FOLLOWED BY AN 'F.' SO WE HAD AN N1 ENG SPD SETTING BASED ON 50 DEGS C OAT VS 50 DEGS F. FORTUNATELY, I NOTICED THE LOWER N1 SETTING AND PUSHED THE THRUST LEVER FARTHER FORWARD, BUT IT TOOK US A WHILE TO DISCOVER WHERE THE PROB WAS AND HOW THE ERROR HAD OCCURRED. I RECOMMEND THAT THE MANUFACTURE IN GRAND RAPIDS, MI ADD A SOFTWARE SAFETY VALVE OR OAT 'REASONABLENESS TEST' THAT WOULD HELP CATCH SUCH ERRORS. ONE WAY TO DO THIS WOULD BE TO FEED ACTUAL OAT OR STATIC AIR TEMP (SAT) FROM THE TAT/SAT/TAS INDICATOR TO THE FMS AND HAVE THE FMS COMPARE MANUALLY ENTERED OAT VALVES VS MEASURED ACTUAL OAT AND ALERT THE CREW VIA AN FMS TEXT ALERT PROMPT IN THE FMS CDU SCRATCHPAD TO THE APPARENT DISCREPANCY. I ALSO RECOMMEND THAT THE FMS SOFTWARE BE CHANGED TO REQUIRE THAT A 'C' OR AN 'F' FOLLOW THE TEMP ENTRY OR ELSE THE COMPUTER WOULD ADVISE 'INVALID ENTRY' FOR ONLY A # ENTRY ATTEMPT ON THE TKOF PAGE.

Data retrieved from NASA's ASRS site as of July 2007 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.