Safety Concern

Air Safety Week, May 9, 2005

The Aviation Safety Reporting System (ASRS) recently sent the following alert to the Airbus airport manager at Kennedy International Airport (JFK):

"ASRS received a captain's report of an incident in which an A319 departing JFK Runway 31L full length experienced dual IRU [inertial reference unit] failures (Numbers 2 and 3) on rotation. The aircraft lost primary flight display and navigation display on the first officer's side of the cockpit, and the flight director on the captain's side. Both autopilots and autothrust were also lost. An emergency was declared and the aircraft returned to JFK for landing. The aircraft reverted to direct law flight characteristics after the landing gear was lowered. Company maintenance informed the flight crew that A319 departures from a specified gate at JFK using Runway 31L full length have had this problem at least a dozen times in the past 2 years. The [submitter of the report] did not speculate on a cause for these incidents, but noted that the flight crew had discussed 'the rough runway or a magnetic mass in the gate vicinity' as possibly being contributory to the loss of IRU function."

The main cause of the problem, it seems, is premature failure of a main subcomponent, the ring laser gyro (RLG). The problem stems from changed manufacturing processes and/or the method of shipment, and it affects a variety of aircraft, not just the A319 (see discussion beginning at p. 116 at www.arinc.com/amc/reports/2004/discuss_items/09_navigation.pdf).

One manufacturer suggests shipping with Shockwatch indicators in order to have a log of abuses (fragile box drops) that occur during shipping.

On the airplane, the design and its shockmounts may not be robust enough. Think about it in terms of eggs. If two IRUs are dependent on RLG's and they fail simultaneously due to shocks to their systems (traversing a pothole or speed bump), it is very similar to what happens when a carton of two eggs is dropped.

In a related matter, ADIRU (air data inertial reference unit) failures seem to dog the Airbus type. A May 3, 2000, airworthiness directive (AD 2000-07- 27) addressed dual critical failures during flight. A Jan. 27, 2004, airworthiness directive (AD 2003-26-03) spoke of modifying the shelf above ADIRU3, the polycarbonate guard and the avionics compartment ladder to prevent failure and loss of critical attitude and airspeed data. The Civil Aviation Safety Authority (CASA) of Australia issued AD/A320/174 (not yet sighted in FAA activity), speaking of these shelf modifications having proved "insufficient" to stop the ADIRU failures (compliance required by May 11).

The air data inertial reference unit is the primary source for attitude, airspeed, and altitude - so it's vital. The attitude and air data signals are formatted for display by the airplane information management system. The ADIRU design differs from the traditional left-right pilot/copilot system partitioning. The ADIRU is a single, fault-tolerant, high-integrity data source for both pilots' primary flight displays (PFD). The ADIRU uses multiple redundant inertial sensors for computing attitude and also selects a best altitude and airspeed from the pitot and static pressure sources. As a result, it provides a single set of data for both the captain and first officer, eliminating cross-channel splits and variance.

The pitot and static pressures are measured by small air data modules (ADM) located as close as possible to the respective pressure sources. The ADMs transmit their pressures to the ADIRU through data buses. In the highly improbable event that the ADIRU's totally fail, a secondary attitude air data reference unit (SAARU) provides comparable attitude and air data to both PFDs. The SAARU also supplies standby attitude directly to an electronic standby horizon instrument. The standby airspeed indicator and altimeter, both electronic, receive pitot and static pressure from the standby ADMs. This design ensures that displayed data are immune to any first and most second failures of their respective sensors or pressure probes. But if the ADIRU's aren't properly mounted, delete all discussion on the subject of reliability. The A380's ADIRU has been extensively flown on A320s for over a year now. That might indicate the level of Airbus concern.

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