TB 1--1520--251--20--03
g. Components/Parts in Work (Depot Level and Others). N/A.
2.
Task/Inspection Suspense Date.
a. Prior to their next flight, unit commanders will brief all assigned AH--64D aviators and ensure that
they are familiar with the procedures contained in this TB.
b. Prior to next flight, post a copy of this TB in TM 1--1520--251--10, TM 1--1520--251--CL, and TM
1--1520--251--MTF.
3.
Reporting Compliance Suspense Date. N/A.
4.
Summary of the Problem.
a. During a day VFR flight, at an altitude of 1000 feet AGL, 143 knots true airspeed (KTAS), an
AH--64D experienced an unexpected pitch--down attitude of approximately 12 degrees with no
corresponding caution, warning or advisory message. Subsequent investigations revealed that during an
in--flight alignment, the EGIs sent differing velocity data signals to the flight management computer (FMC)
which resulted in the FMC inappropriately scheduling the stabilator to approximately 25 degrees trailing
edge down position.
b. For manpower/downtime and funding impacts. N/A.
c.
The purpose of this TB is to:
(1) Provide flight crews with additional information relative to this incident.
(2) Provide additional emergency procedures, notes, and cautions to TM 1--1520--251--10
and TM 1--1520--251--CL.
5.
End Items To Be Inspected.
N/A.
6.
Assembly Components To Be Inspected. N/A.
7.
Parts To Be Inspected. N/A.
Inspection Procedures. N/A.
Correction Procedures. (Relative Information)
a. Extensive testing by Boeing has concluded that the AH--64D flight controls system, including the
stabilator, can be adversely impacted by erroneous data from the EGI. If the stabilator were slewed down
in response to an erroneous velocity generated by the EGI, that velocity would be displayed on the PDS
and HDU. As such, the same inappropriate velocity that caused the stabilator to slew down, also allows
the pilot to take manual control of the stabilator and return it to an appropriate position.
b. The EGIs in the AH--64D provide attitude, rate and velocity information to the FMC and other
systems. In this instance both EGIs aligned with very poor position confidence values, even though both
EGIs were tracking satellites. The automatic EGI alignment mode failed and the attempted manual reset
mode also failed. The indications to the crew that the EGIs were not aligned properly were the poor
position confidence values displayed on the TSD utility page, waypoint and route information was not
displayed on the TSD utility page, waypoint and route information was not displayed on the TSD, and an
update position advisory message was displayed on the UFD. Just prior to the incident, displayed
airspeed began to change rapidly and erratically (143 KTAS to 258 KTAS within one second then
decreased back to 143 over a period of 10 seconds). Because neither EGI detected a fault during the
continuous built in test sequence, no indication of failure was presented to the crew. The erroneous
velocity, attitude, and position errors were passed directly to the system processor and flight
management computer (FMC).
c. The AH--64D flight controls system, including the stabilator, can be adversely impacted by
erroneous data from the EGI. It is critical that the crew understands that erronoeus attitudes, velocities
and rates generated by an improperly aligned or operating EGI will pass those errors to the flight control
system, the displays and the FCR. The pilot must insure that both EGIs have aligned properly and are
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