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A torching APU start occurs when leftover fuel from a previous unsuccessful start attempt does not drain from the APU properly and ignites during a subsequent start attempt. When a torching start occurs, the accumulated fuel in the APU tailpipe is consumed and the APU operation is otherwise normal. If unburned fuel mist is blown back onto the empennage surfaces during the initial unsuccessful start attempt, it is possible that a fire on the external surfaces of the empennage could occur if torching occurred during the next start attempt.
The only means to detect the torching start and/or flames on the empennage surfaces is by an external observer. By the time the observer communicates to the
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Flight Crew Operations Manual Bulletin No. ELF-15,dated June 16, 1995 (continued)
crew that a torching start has occurred, the excess fuel will most likely be consumed and the torching ceased. Unless the operator sees the evidence that a fire exists on the empennage surface, no other flight crew action is required except for a normal APU shutdown to allow the required inspections of the airplane surfaces.
If the observer sees fire on the airplane surfaces, the flight crew should advise the tower and request fire equipment. In this instance, the APU can be shut down either by normal procedures since the APU fire extinguishing system would not be effective to combat either the APU torching or the external surface fire. Inflight starting of the APU is not impaired because the fuel vapors are carried away from the airplane. Torching of any leftover fuel in the APU exhaust area will not damage the airplane.
The Federal Aviation Administration (FAA) issued an Airworthiness Directive (AD) effective March 12, 1990 requiring that after an unsuccessful ground start the APU be placarded to prohibit ground operation or that any subsequent APU ground start attempts be monitored by a “qualified ground observer.”.
The Boeing Company designed a modified system to improve draining of leftover fuel after an unsuccessful APU start. These modifications are described under Administrative Information below.
Operating Instructions
For airplanes with unmodified APU drain systems, the following procedures apply:
1.
Following any unsuccessful APU start attempt, the subsequent APU ground start attempt(s) must be monitored by a qualified ground observer to assure that the airplane is not damaged due to torching.

2.
The placard may be removed and APU ground starting resumed without an observer following appropriate maintenance action to determine and resolve the cause of the unsuccessful ground start, or successful ground or inflight starting and operation is accomplished.

NOTE: Inflight starting and operating of the APU is not impacted by this action.
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Flight Crew Operations Manual Bulletin No. ELF-15,dated June 16, 1995 (continued)
Administrative Information
Insert this bulletin behind the FCOM Bulletin Record Page in Volume 1 of your FCOM. Amend the FCOM Bulletin Record Page to show Bulletin ELF-15 "IN EFFECT" (IE)
This Operations Manual Bulletin will be cancelled after Boeing is notified that all affected airplanes in the operator’s fleet have been modified by one of the following methods:
1.
Installation of a Garrett GTCP 85-129 APU with PRR 33890-86 incorporated (installs a modified drain system on airplanes at production line number 20161 and on).

2.
Incorporation of Service Bulletin 737-49-1073 (installs the modified drain system on airplanes delivered prior to incorporation of PRR 33890-86).

3.
Installation of the Sundstrand APS 2000 alternative APU (includes the modified drain system).

4.
Installation of the Garrett GTCP 36-280 alternative APU (includes the modified drain system).

The FAA has approved the above four options as acceptable means of compliance to the above Airworthiness Directive. If the operator does not plan to modify all of the airplanes and would like to have the content of this Bulletin incorporated in the Operations Manual, please advise Boeing accordingly.
Please send all correspondence regarding FCOM bulletin status to one of the following addresses:
Mailing Address:  Manager 737, Flight Technical Integration & Data Boeing Commercial Airplane Group
P. O. Box 3707 MS 20-89 Seattle, WA 98124-2207 USA
Fax Number: (206) 662-7812
Telex: 329430 Station 627
SITA: LKEBO7X
E-MAIL: FTID@Boeing.com

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Flight Crew Operations Manual Bulletin No. ELF-15,dated June 16, 1995 (continued)
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Flight Crew Operations Manual Bulletin
for
 

The Boeing Company Seattle, Washington 98124-2207

737
Number: ELF-16 Date: June 16, 1995 Document Effectivity: 
Subject:  BLEED TRIP OFF Light Illuminating During A No Engine Bleed Takeoff 
　
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