Narrative:

Departed ZZZ for XXX with 4000 ft of runway available at ZZZ. Takeoff field length calculations had been done by previous duty crew and I discussed them with the previous duty captain and confirmed they were correct. I discussed with the other captain the likelihood we would have to return with patient to XXX because of limited runway at ZZZ. When flight was called and dispatched, I rechked numbers, calculated from performance charts in checklist that we would require less than 2800 ft to land upon our return to ZZZ and using 2 different calculators concluded that met the part 135 60% rule. During our wait for patient at XXX, (again calculated landing distance of less than 2800 ft and again miscalculated that as 60% of the 4000 ft available). I showed my calculations to my copilot and confirmed with operations that we would be fine with only 4000 ft at ZZZ. On landing at ZZZ we taxied off of runway at taxiway A11 with little braking required. That left over 1100 ft of runway remaining. The runway closure was at the same location (abeam the south edge of taxiway A8, with red tape along the runway edges for approximately 200 ft and a large, elevated lighted 'X' on the centerline. 2 flashes of illumination struck me as I wrote the above. The charted length from taxiway A8 to the end is 5000 ft, and we were at 50 ft RA as we crossed the line across the runway signifying the closed portion. I caught the error in my calculations while reviewing my paperwork after the flight. We did, indeed, stop within 60% of the available runway without extraordinary plting effort, and I do not believe anyone was subjected to any additional hazard. I have recalculated the numbers I used for the decision making process, and was able to duplicate my erroneous outcome by multiplying by .7 vice the required .6. I believe I am not likely to repeat that many keystroke errors ever again.

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Original NASA ASRS Text

Title: LTT AIR AMBULANCE CAPT MADE A COMPUTATION ERROR IN CALCULATING LNDG PERFORMANCE DISTANCE REQUIRED.

Narrative: DEPARTED ZZZ FOR XXX WITH 4000 FT OF RWY AVAILABLE AT ZZZ. TKOF FIELD LENGTH CALCULATIONS HAD BEEN DONE BY PREVIOUS DUTY CREW AND I DISCUSSED THEM WITH THE PREVIOUS DUTY CAPT AND CONFIRMED THEY WERE CORRECT. I DISCUSSED WITH THE OTHER CAPT THE LIKELIHOOD WE WOULD HAVE TO RETURN WITH PATIENT TO XXX BECAUSE OF LIMITED RWY AT ZZZ. WHEN FLT WAS CALLED AND DISPATCHED, I RECHKED NUMBERS, CALCULATED FROM PERFORMANCE CHARTS IN CHKLIST THAT WE WOULD REQUIRE LESS THAN 2800 FT TO LAND UPON OUR RETURN TO ZZZ AND USING 2 DIFFERENT CALCULATORS CONCLUDED THAT MET THE PART 135 60% RULE. DURING OUR WAIT FOR PATIENT AT XXX, (AGAIN CALCULATED LNDG DISTANCE OF LESS THAN 2800 FT AND AGAIN MISCALCULATED THAT AS 60% OF THE 4000 FT AVAILABLE). I SHOWED MY CALCULATIONS TO MY COPLT AND CONFIRMED WITH OPS THAT WE WOULD BE FINE WITH ONLY 4000 FT AT ZZZ. ON LNDG AT ZZZ WE TAXIED OFF OF RWY AT TXWY A11 WITH LITTLE BRAKING REQUIRED. THAT LEFT OVER 1100 FT OF RWY REMAINING. THE RWY CLOSURE WAS AT THE SAME LOCATION (ABEAM THE S EDGE OF TXWY A8, WITH RED TAPE ALONG THE RWY EDGES FOR APPROX 200 FT AND A LARGE, ELEVATED LIGHTED 'X' ON THE CTRLINE. 2 FLASHES OF ILLUMINATION STRUCK ME AS I WROTE THE ABOVE. THE CHARTED LENGTH FROM TXWY A8 TO THE END IS 5000 FT, AND WE WERE AT 50 FT RA AS WE CROSSED THE LINE ACROSS THE RWY SIGNIFYING THE CLOSED PORTION. I CAUGHT THE ERROR IN MY CALCULATIONS WHILE REVIEWING MY PAPERWORK AFTER THE FLT. WE DID, INDEED, STOP WITHIN 60% OF THE AVAILABLE RWY WITHOUT EXTRAORDINARY PLTING EFFORT, AND I DO NOT BELIEVE ANYONE WAS SUBJECTED TO ANY ADDITIONAL HAZARD. I HAVE RECALCULATED THE NUMBERS I USED FOR THE DECISION MAKING PROCESS, AND WAS ABLE TO DUPLICATE MY ERRONEOUS OUTCOME BY MULTIPLYING BY .7 VICE THE REQUIRED .6. I BELIEVE I AM NOT LIKELY TO REPEAT THAT MANY KEYSTROKE ERRORS EVER AGAIN.

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.