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Boeing recently received reports from two 747-400 operators regarding low-time
removal of override/jettison pump motor/impeller assemblies installed in main
tanks 2 and 3, and center wing tanks, due to low fuel pressure indications. To date,
Boeing has received reports of twelve low-time removals of override/jettison
pumps, all of which had accumulated less than 700 airplane flight hours. Visual
inspection of the removed pumps revealed damage to the inducer and impeller,
with related score marks on the inner diameter of the inlet adapter. This sustained
metal-to-metal contact is a possible ignition source. Testing by the pump
manufacturer, Hydro-Aire, did not result in ignition. The FAA has issued
Airworthiness Directive T98-25-52.
Page 2 of 6
Operations Manual Bulletin No. ILF2-41(R5), Dated April 1, 1999 (continued)
The following procedures apply to pumps manufactured, overhauled, or repaired
between July 1996 and November 1998. Operators can determine specific airplane
applicability through methods published in FAA letter 98-140S-440, “Approval of
AlternativeMethod of Compliance with Airworthiness Directive T98-25-52”, and
respective Service Bulletins.
If CWT override/jettison pumps are to be used for flight, the center wing tank must
contain at least 17,000 pounds (7,720 kilograms) prior to engine start. This
ensures both CWT override/jettison pumps remain covered during rapid
acceleration and high nose attitudes during takeoff and climb.
If CWT fuel is less than 50,000 pounds (22,700 kilograms) prior to engine start,
the flight crew should select the CWT override/jettison pumps OFF when fuel in
the center wing tank reaches 7,000 pounds (3200 kilograms). This ensures a
potentially damaged pump remains covered by fuel, and is not exposed to fuel
vapors during climb. The flight crew may select the CWT fuel pumps ON when
stabilized in cruise, then select the CWT fuel pumps OFF when CWT fuel reaches
3,000 pounds (1,360 kilograms).
Airplanes with electric CWT scavenge pumps will scavenge some or all of the
remaining 7,000 pounds (3,200 kilograms) of fuel. However, this scavenging will
not occur until the reserve tanks begin to transfer. Based on a minimum scavenge
flow rate of 1,700 pounds per hour, approximately 3,400 pounds (1,540
kilograms) of fuel will be scavenged within the two hour scavenge pump
activation cycle. The remaining 3,600 pounds (1,630 kilograms) is unusable. The
scavenge flow rate is a function of several variables (pump efficiency, discharge
pressure, etc.). On some airplanes, the majority of the 7,000 pounds (3,200
kilograms) of CWT fuel may be scavenged within the two hours. Operator
experience may provide different values for scavenged and unusable fuel.
All of the remaining 7,000 pounds (3,200 kilograms) of CWT fuel is unusable in
airplanes with CWT jet scavenge pumps. However, in a low fuel situation, the
CWT fuel pumps may be selected ON and all CWT fuel used.
If CWT fuel is greater than or equal to 50,000 pounds (22,700 kilograms) prior to
engine start, the crew should select the CWT override/jettison pumps OFF when
CWT fuel reaches 3,000 pounds (1,360 kilograms). Increased CWT fuel ensures
both CWT override/jettison pumps remain covered during rapid acceleration and
high nose attitudes during takeoff and climb. Under these conditions, CWT pump
shutoff is not required until the cruise phase of flight.
On airplanes with the electric CWT scavenge pump, all of the remaining 3,000
pounds (1,360 kilograms) of fuel will be scavenged and can be considered usable
fuel.
Page 3 of 6
Operations Manual Bulletin No. ILF2-41(R5), Dated April 1, 1999 (continued)
On airplanes with jet scavenge pumps, some or all of the remaining fuel may not
scavenge. With 3,000 pounds (1,360 kilograms) of fuel remaining in the center
wing tank, it is likely the jet scavenge pumps will activate, but not until later in the
flight when there may not be sufficient time to scavenge all remaining fuel. It will
take approximately 2 hours to scavenge 3000 pounds (1,360 kilograms) of fuel.
Initially, the 3000 pounds of fuel should be considered unusable. Operator
experience may provide a different value of unusable fuel.
In the above cases, the 7,000 pounds (3,200 kilograms) or 3,000 pounds (1,360
kilograms) of CWT fuel would normally be considered as “fuel in lieu of payload”
as described in the Weight and Balance Manual, Section 1-22-01. Boeing has
reviewed the certification conditions for the 747-400 and determined for AD T98-
25-52 only, CWT fuel up to 7,000 pounds (3,200 kilograms) need not be
considered as fuel in lieu of payload.
Boeing Service Bulletin 747-28-2226 (December 22, 1998) provides an alternate
　
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