PHOENIX, AZ, USA
N737AV
CESSNA 172N
MECHANICS IN A BOEING 737 MADE A HIGH SPEED TAXI DOWN RUNWAY 26L TO TEST THE ENGINES AS CESSNA 172N, N737AV, WAS ON FINAL APPROACH TO LAND. THE TOWER CONTROLLER TOLD THE BOEING MECHANICS TO EXIT AT MIDFIELD AND 'CONTINUE IN THE TURN ONTO THE PARALLEL TAXIWAY NO DELAY OFF PLEASE.' ONE MINUTE LATER, THE MECHANICS ASKED TO GO TO THE GATE AND THE CONTROLLER SAID TO CONTACT GROUND CONTROL. AFTER TURNING ONTO THE PARALLEL TAXIWAY, THE BOEING CAME TO A STOP. THE MECHANICS DID NOT CONTACT GROUND CONTROL UNTIL AFTER THE ACCIDENT (1 MINUTE 45 SECONDS LATER). THE TOWER CONTROLLER TOLD THE CESSNA PILOT TO EXIT AT THE SAME TAXIWAY, THEN CONTACT GROUND CONTROL. HE SAID HE CLEARED THE CESSNA ON THE ASSUMPTION THAT THE BOEING WOULD NOT STOP. THE CESSNA 172 EXITED THE RUNWAY AND STOPPED AT THE HOLD SHORT LINE. THE CESSNA WAS TOLD TO TAXI NORTHBOUND CROSSING BEHIND THE BOEING. THE CESSNA MOVED JUST AS THE BOEING BEGAN POWERING UP TO MOVE AND THE JET BLAST RESULTED IN SUBSTANTIAL DAMAGE TO THE CESSNA. THE MECHANICS SAID THEY STOPPED AFTER THE TURN AND SET THE BRAKES WHILE TUNING THE RADIO TO GROUND. THEY SAID THEY POWERED UP TO 40 TO 50 PERCENT N1 TO BEGIN TAXIING AGAIN WHEN THEY HEARD ABOUT 'BLASTING THE CESSNA.' N1 AT IDLE POWER WAS 21%. BOEING SPOKESMAN SAID BREAK-AWAY THRUST FOR AN EMPTY AIRCRAFT WAS 25 PERCENT N1, WHILE A FULLY LOADED AIRCRAFT NEEDED 35%.
On April 6, 1995, at 1347 mountain standard time, a Cessna 172N, N737AV, was blown over while taxiing behind a Boeing 737-500 at the Phoenix Sky Harbor International Airport, Phoenix, Arizona. The aircraft was owned and operated by the pilot and was on a personal cross-country flight. Visual meteorological conditions prevailed and an instrument flight rules flight plan was filed. The aircraft sustained substantial damage. The certificated private pilot and his one passenger were not injured. The flight originated at Fullerton, California, on the day of the accident at 1130 Pacific daylight time as a flight to Phoenix. Written and oral statements were obtained from the pilot of the Cessna 172, the two mechanics operating the Southwest Airlines Boeing 737, and controllers on duty in the Federal Aviation Administration (FAA) Air Traffic Control Tower (ATCT). In addition, transcripts of the air-to-ground communications tapes were reviewed. The transcripts, statements, and a map of the airport are attached to this report for reference. The Boeing 737 was operated by two mechanics who were to perform an engine run-up to takeoff power in order to check the compressor low bleed pressure on the number one engine. The ATCT cleared the Boeing to do a high speed taxi on runway 26L at 1342. During this time, the Cessna 172 was on downwind and was cleared to land on the runway at 1341. By 1343, the ATCT advised the Boeing that they would have to exit the runway midfield at taxiway E8 due to landing traffic (the Cessna). The Boeing was given a clearance to "continue in the turn onto the parallel taxiway (Delta) no delay off please traffic is very short final." One minute later, the Boeing crew transmitted at 1344 to the local controller, "Uh we're just gonna head back to the gate." The local controller responded "Southwest 521 why don't you contact ground point niner." According to witness statements, after turning onto Delta the Boeing came to a stop. The communications transcript reveals that the Boeing does not contact ground until after an unknown aircraft transmits that "Southwest is blasting that Cessna" at 1346:15. While the Boeing was taxiing on the runway, the Cessna 172 was on final approach and landed as the Boeing was exiting the runway. The local controller instructed the Cessna 172 to exit the runway at E8, then contact ground control. The local controller reported that he predicated the instruction to the Cessna on the assumption that the Boeing would not stop on the taxiway. As the Boeing came to a stop on Delta, the Cessna 172 exited the runway at E8 and stopped at the hold short line. The transcript shows that the Cessna contacted ground control at 1345:08 and was given a clearance to taxi northbound on Sierra (crossing perpendicular to Delta behind the Boeing). The Cessna pilot acknowledged the clearance at 1345:45. According to verbal reports from the controllers, the Cessna 172 moved a little toward Delta just as the Boeing began powering up to move. The jet blast from the engines picked the Cessna 172 up and deposited it on its side. In an oral statement, the Cessna 172 pilot reported that he was aware of, and concerned about, the Boeing 737 on the runway as he was on final approach. The pilot heard the clearances given to the Boeing, and watched it exit the runway at E8 as he was touching down. The pilot reported that he was uncomfortable with the controllers instruction to exit the runway at E8 and came to a stop near the runway edge. After a conversation with ground control and receiving the clearance to Sierra, the pilot moved his aircraft forward and encountered the jet blast. In written statements, the mechanics taxiing the Boeing said that on the runway the engines were set to power for a few seconds, and then they heard the controller instruct them to exit the runway about halfway down for landing traffic. They reported that they then pulled the power back to 60 percent N1, released the brakes and began taxiing. They state that they recall the controller's instruction "to exit at E8 and hold while a United 737 to turn on Sierra northbound." The mechanics reported that they then powered up to turn onto Delta, stopped, and set the brakes while they set the radio to ground control. They then said the aircraft was powered up to an estimated 40 to 50 percent N1 to begin taxiing again when they heard the transmission about "blasting the Cessna." Boeing was contacted by telephone and subsequently provided the following estimated thrust values. Idle N1 is about 21 percent. Break-away thrust to begin taxiing an empty aircraft requires a momentary advancement from idle to about 25 percent N1, then back to idle for the remainder of the taxi. Break-away thrust for a fully loaded aircraft is 35 percent.
THE USE OF EXCESSIVE BREAK-AWAY THRUST BY THE BOEING 737 CREW. A FACTOR RELATED TO THE ACCIDENT WAS: IMPROPER PLANNING/DECISION BY THE CESSNA 172 PILOT BY ACCEPTING A CLEARANCE TO TAXI BEHIND THE BOEING 737 WITHOUT ASSURANCE THAT THE BOEING 737 CREW WOULD NOT POWER UP.
Source: NTSB Aviation Accident Database
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