85-11-51 R1 SAAB-FAIRCHILD: Amendment 39-5145 as amended by amendment 39-5857. Applies to all Saab-Fairchild Model SF-340A airplanes certificated in any category.
Compliance is required before further flight after the effective date of this airworthiness directive (AD).
To prevent incorrect attitude indications, accomplish the following, unless previously accomplished:
A. Incorporate the following information into the limitations section of the FAA- approved Airplane Flight Manual and provide to flight crews:
"During the alignment or initialization period, an inertial system is susceptible to bus voltage transients and aircraft movement. The method traditionally used to initialize an inertial system is to apply power to the system and to keep the aircraft stationary until all errors in the system are biased to zero. Aircraft movement due to taxiing will cause inertial errors that are excessive.
The following procedures must be used to correctly initialize inertially-base attitude heading reference systems (AHRS) to establish the correct attitude and heading references with respect to earth references:
1. AHRS initialization to be performed with both engines running, i.e., external power switched to off and both generators on line prior to applying power to the L and R avionics busses.
2. Approximately 70 seconds after avionics power is applied, the AHRS initialization is completed by the presentation of attitude on EFIS and the removal of the attitude flags from the displays. During initialization, ensure that the aircraft is not moved and there is no operation of brakes, flaps, nosewheel steering, i.e., the hydraulic pump is not to be operated. Also, there should be no changes made in engine power/prop settings.
3. After the system is initialized, as indicated by the attitude flag being out of view, the aircraft may be taxied and engine run-ups performed. Takeoff may not be made until the system has been operating atleast two minutes after initialization is completed, the attitude difference between the attitude displayed on both EFIS electronic attitude direction indicators (EADI's) and the standby attitude indicator is 3 degrees or less (either bank or pitch), and the heading on the compass card is not slewing away from the aircraft heading.
4. If the attitude error exceeds 3 degrees or if the compass heading slews away from the aircraft heading when checked in accordance with step 3., above, stop the aircraft, set brakes, stabilize engine power settings, and remove avionics power from the affected system by pulling the AHRS circuit breakers (AHC Avion and BAT) which will remove the attitude display from the EFIS. Reset circuit breakers and repeat initialization procedure and checks as in steps 2 and 3, above.
NOTE: An incorrect initialization on the ground cannot be corrected by a reinitialization while airborne."
B. "Accomplishment of Modification 1438 in accordance with SAAB Service Bulletin SF 340-34-038, dated October 24, 1986, or an equivalent production change constitutes terminating action for requirements of paragraph A. of this AD. Thereafter, the AHRS initialization shall be accomplished in accordance with SAAB Aircraft Operations Manual (AOM) Bulletin Number 24. A copy of AOM Bulletin Number 24 must be readily available to the crew during operations."
C. Alternate means of compliance which provide an acceptable level of safety may be used when approved by the Manager, Standardization Branch, ANM-113, FAA, Northwest Mountain Region.
NOTE: Compliance with paragraph A. of this directive may be effected by including a copy of this AD in the limitations section of the FAA-approved airplane flight manual and operating manual.
All persons affected by this directive who have not already received these documents from the manufacturer may obtain copies upon request to Saab-Fairchild Product Support, S.58188, Linkoping, Sweden. These documents also may be examined at the FAA, Northwest Mountain Region, 17900 Pacific Highway South, Seattle, Washington or the Seattle Aircraft Certification Office, 9010 East Marginal Way South, Seattle, Washington.
Amendment 39-5145 became effective October 21, 1985. It was effective earlier to all recipients of telegraphic AD 85-11-51 issued May 31, 1985.
This Amendment, 39-5857 becomes effective April 6, 1988.