Landing Gear
Proper servicing of landing gear is obviously important to ensure proper landing gear operations during takeoff and landing. It is equally important to ensure proper retraction and extension to prevent potential interference with other aircraft systems in the case of abnormal landing gear conditions.
If the landing gear servicing tasks are not properly performed, issues can occur such as struts seized in a retracted position and strong vibrations that can affect the function of avionics equipment
Case study:
Ten minutes after takeoff, passing FL340,
the flight crew of an A319 lost the autopilot and the autothrust. The AUTO FLT
AP OFF, AUTO FLTA/THR OFF and ENG THRUST LOCKED ECAM alerts were triggered. The
Flight Directors were no longer displayed on the PFD. The NAV FM/GPS POS
DISAGREE ECAM alert was briefly triggered twice, but this was not seen by the
flight crew. The flight crew managed to reengage the AP and the ATHR 5 minutes
later. The flight crew noticed abnormal IRS positions on the MCDU position
monitor page. They decided to continue the flight with the support of ATC to
assist them with determining their position.
During the ILS approach, the AP and ATHR disconnected again at 4000 ft. The flight crew discontinued the approach. They decided to perform a manual approach using only radio navigation aids and they safely landed the aircraft.
Recorder data analysis showed that the AP and ATHR disconnected due to severe drift of the 3 IRS. The first IRS was rejected by the Auto Flight System (AFS) during the climb and the AFS: ADIRU 1/2/3 DISAGREE PFR maintenance message was triggered with no operational impact. A discrepancy between the 2 remaining IRS in the following few minutes led to the rejection of both IRS and to the loss of the AP and ATHR. This triggered the AUTO FLT AP OFF and AUTO FLT A/THR OFF ECAM alerts. The IRS drift then decreased and remained stable during the flight, but it increased again during approach, causing the second loss of AP and ATHR.
The IRS drift started during the takeoff roll. The analysis showed that the root cause was abnormal shocks and high vibrations transmitted to the 3 Air Data Inertial Reference Units (ADIRUs) by the Nose Landing Gear (NLG) during the takeoff roll. This forced the IRS to operate outside of its qualification envelope and it caused the IRS drift condition.
Effects of incorrect shock absorber servicing:
The NLG shock absorber was overinflated during its last service. This made the shock absorber stiffer and reduced its ability to absorb impacts and vibrations. The vibrations experienced during this event were transmitted through the shock absorber to the aircraft structure. The A320 family aircraft ADIRUs are installed in the avionics bay aft of the NLG bay, and they were affected by the excessive vibrations and shocks during the takeoff roll on this flight.