A Very Halting Affair to Remember

Air Safety Week, Feb 28, 2005

A320 Braking Failures Implicate Control Unit

Braking problems with the Airbus A319/A320 and A321 family are officially on the radar screen. Five incidents in three years in the United Kingdom involving a loss of braking after touchdown are bringing safety concerns to the forefront. The UK Aircraft Accident Investigation Board (AAIB) has requested Airbus provide an automated warning to crews on the loss of braking effectiveness after touchdown or rejected takeoff.

The hazards involved are very real -- as evidenced by the overrun accident of Leisure International Airways A320 (registration G-UKLL) at Ibiza Airport (IBZ) in Spain on May 21, 1998, and the near overrun of a Skyservice Airlines A320 (reg. C-FTDF) at Cardiff Airport in the UK on Aug. 3, 2003.

The AAIB just released the details of the Canadian registered Airbus A320 of Skyservice Airlines. On finals to Runway 30, the Electronic Centralized Aircraft Monitoring (ECAM) display showed a STEERING caption. The pilot cycled the Anti-SKID & N/Wheel STRNG switch in an attempt to reset the Brake and Steering Control Unit (BSCU). It appeared to have successfully reset but after touchdown the aircraft did not decelerate normally under auto-braking.

The pilot depressed the brake pedals fully but no deceleration was felt. He then selected maximum reverse thrust and the copilot cycled the A/SKID & N/W STRNG switch. The pilot again attempted toe-braking but without any effect, so the crew selected the A/SKID & N/W STRNG switch to OFF. The commander was then able to brake effectively to bring the aircraft to a halt about 130 feet (40 meters) from the end of the runway, bursting three mainwheel tires and damaging a landing-gear light. There had been no warning at all on the ECAM and so the captain, due to his gentle braking inputs, had taken between 10 and 13 seconds to realize that the BSCU had in fact failed.

The Incident

For noise abatement, the captain had decided to use idle reverse only and LOW on the Autobrake, the runway being adequately long. The approach was uneventful until, passing 1,000 feet, the aircraft's status page changed from Cat III DUAL to CAT III Single. This downgrade meant that any single system failure would terminate the automatic approach. Simultaneously, an amber STEERING caption was noted on the ECAM's WHEEL page. A cycling of the A/SKID & N/W STRNG extinguished the caption and a restored status of Cat III DUAL then showed. Neither pilot could recall re-selecting autobrake after cycling the switch. After touchdown and idle reverse selection, the copilot noted that the autobrake was not functioning and called out "Manual Braking." The pilot selected full toe-braking, but gingerly and over a period of 10 seconds. Eventually recognizing "no joy on the braking front," he applied full reversing and instructed the copilot to cycle the A/SKID & N/W STRNG switch. This had nil effect, so he ordered the switch turned OFF in order to access stored hydraulic pressure in the accumulator. Braking was now available, and he urgently brought the aircraft to a halt. With three tires burst and a fourth damaged, the runway was blocked until the tires were changed.

The Systems

The A/SKID & N/W STRNG switch removes anti-skid protection requiring the pilot to refer to the triple pressure gauge in order to keep toe-braking pressures below 1,000 psi and not blow tires. The A320 brakes operate off normal GREEN system with the Alternate YELLOW system using stored pressure. The BSCU is a two-channel computer that controls anti-skid and autobrake functions (the latter being MAX/Med or Low). In addition to Normal braking (autobrake with anti-skid) there are three other modes:

a. Park Brake (ON or modulated cautiously) -- the last-ditch nondifferential unsteered option;

b. Alternate braking with anti-skid (toe-pedal operation with anti-skid); and

c. Alternate braking without anti-skid (pedal-braking due to BSCU failure or A/SKID & N/W STRNG selected to OFF).

Manufacturer Airbus could not replicate the fault codes recorded by the BSCU BITE (built-in test), the CFDS (Central Fault and Display) nor the flight data recorder (FDR). However, very brief "micro-cut" power interruptions revealed a problem in the separate power supplies for the two BSCU channels. The FDR disclosed that the cycling of the A/SKID & N/W STRNG on finals had caused a swap-over in the active BSCU channels and a consequent silent loss of autobrake arming. In a word, "tricky." After touchdown, the spoilers had extended and reverse operated, but due to lack of auto-braking the ineffectiveness of these two devices at lower speeds quickly caused the rate of deceleration to drop off from its peak of 0.18g. Nineteen seconds after touchdown, the pilot's selection of max reverse brought the deceleration back up to 0.19g only. Effective longitudinal deceleration, peaking at 0.4g, only became apparent 28 seconds after touchdown -- but three sharp spikes on the FDR noted the rapidly resultant tire-bursts. The aircraft came to rest 50 seconds after touchdown. Data loss from the BSCU was noted 23 seconds after touchdown (equating to the copilot's cycling of the switch).