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Networks and distributed Systems
Research group of Prof. Peter B. Ladkin, Ph.D.
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A320 Attitude Control Input and
DC-10 Autobrake System Anomalies

Excerpts from (Australian) Bureau of Air Safety Report B/916/3032

exerpted by Peter Ladkin

Technische Fakultät, Universität Bielefeld

16 October 1996


On Monday 12 August 1991, at 1023 hours Eastern Standard Time (EST), a McDonnell Douglas DC-10 Series 30ER aircraft (DC-10) operated by Thai Airways International was landing on runway 34 at Sydney (Kingsford Smith) Airport. The DC-10 was carrying 185 persons. At the same time, an Airbus A320-211 aircraft (A320), operated by Ansett Australia was on a short final approach for landing on runway 25. The A320 was carrying 110 persons.

Runways 34 and 25 intersect, and Simultaneous Runway Operations (SIMOPS) were in progress.

Landing instructions to the crew of the DC-10 included a requirement for the aircraft to be held short of the intersection of runways 34 and 25.

A Quantas Airways Boeing B747 aircraft was holding on taxiway Victor ('V'), north of runway 25 and west of runway 34, awaiting the landing of the A320 and a subsequent clearance to cross runway 07/25. The B747 was carrying 372 persons.

While observing the DC-10's landing roll, the captain of the A320 judged that the DC-10 might not stop before the intersection of the runways. He elected to initiat a go-around from a low height above the runway.

Under heavy braking, the DC-10 slowed to about 2 kts ground speed, at which time the nose of the aircraft was approximately level with the edge of runway 07/25.

During the go-around executed by the crew of the A320, that aircraft passed above the DC-10 on its left and the B747 on the right of its flight path.

1.19 Aircraft Systems Information

During the course of the investigation it became apparent that anomalies existed with regard to the attitude control inputs on the A320, and in the braking system of the DC-10.

1.19.1 Sidestick Controllers -- A320

The first officer indicated that he was in no doubt that the captain was taking control of the aircraft. He relaxed his grip on the right sidestick, but did not remove his hand. He stated that he was not aware of making any subsequent intentional control inputs through his sidestick. However, the DFDR readout indicated that neutral and nose-down inputs were made for some 12 s. The inputs from the first officer's side stick did not detract from the captain being able to achieve the desired aircraft attitude.

The two sidesticks of the A320 are essentially independent insofar as the pilots are concerned, in contrast to the 'traditional' system in which the two control colimns in the cockpit are mechanically interconnected. In other words, while the A320's computer systems co-ordinate the inputs from both sidesticks and base the control response on the algebraic sum of the inputs, there is no linkage between the two with regard to control feel. As a result, the inputs being made by each pilot on his sidestick cannot be sensed through his sidestick by the other. Had there been such a sense of movement between the two sidestick controllers, the co-pilot could have sensed the captain's input as he initiated the go-around, and released any pressure on his sidestick.

The A320 design makes provision for either pilot to take full control with his sidestick, e.g. in the event that one pilot should become incapacitated. To assume priority for his sidestick, i.e. to direct the computers to ignore inputs from the other sidestick, the pilot who wishes to assume priority must activata the 'instinctive autopilot disconnect button', more commonly referred to as the 'take-over button'. As soon as this button is activated, control authority is immediately transferred to that sidestick. However, the button has to be held down continuously for 30 s before control priority is permanently reallocated to that sidestick. Activation of the take-over button on a go-around was not part of Ansett Australia's standard operational procedures.

Ansett Australia's procedures for activation of the take-over button (viz. to do so when it is believed that the response from the side stick is not normal), follow the recommendations and standard operating procedures laid down in the aircraft manufacturer's manual. As the aircraft was achieving the attitude required by the captain, he saw no requirement to activate the button.

1.19.2 Auto brake system -- DC-10

The captain of the aircraft indicated that the auto brake system was set to 'MIN' (minimum) for the landing, but that a malfunction of the system occurred.

Information provided by McDonnell-Douglas and Thai Airways International indicated that, provided the MIN landing mode had been selected by the crew, the auto brake system fitted to this DC-10 should have commenced to function 4 s after the deployment of the aircraft's ground spoilers. These are activated with main gear spin-up when the flaps are in the landing range (more than 30°). The DFDR readout showed that wheel braking did not actually commence until 10.24.02 EST, some 23 s after the main wheels had touched down. This delay is consistent with the auto brake malfunction reported by the crew. However, the auto brake system is designed so that if a malfunction doe occur, the system will automatically disarm, the arm-disarm switch will move to DISARM, and indicating lights will come on. [ be continued when I receive the rest of this paragraph. PBL]

2.2.5 A320 Crew

As part of Ansett Australia Airlines' procedures for one pilot to take control of an aircraft from the other pilot, the pilot taking control is required to say 'taking over' or 'I've got it'. This procedure is designed to ensure that the pilot relinquishing control of the aircraft does so in a timely and positive manner. In this incident the captain of the A320 said 'Going around' and not 'Taking over', or 'I've got it'.

While the first officer reported that he was not aware of any delay or confusion in the hand-over/take-over process resulting from the captain's use of non-standard terminology, it is possible that the sequence was imperceptibly delayed for a short period of time.

2.2.6 Aircraft performance -- A320

Although the A320 successfully avoided the DC-10, under different circumstances the cross controlling between the two pilots (see para. 1.18.2) could have jeopardised a safe go-around.

The DFDR readout indicated that as the A320 commenced its go-around, both crew members were manipulating their sidestick controllers. Approximately 12 s of dual control inputs were recorded. The captain was making pitch-up inputs while the FO was alternating between neutral and pitch-down inputs. However, the FO stated that he was not consciously aware of having made any control inputs following the captain's call of `Going Around'. As described earlier, the aircraft's computer systems based the control response on the algebraic sum of the two sidestick inputs, as neither had priority.

It is evident that crew co-ordination broke down somewhat as the go-around was initiated, because the company standard hand-over/tak-over procedure was not employed. This may have contributed to the short period of control inputs from both pilots. This simultaneous input situation would almost certainly have been immediately apparent, and corrected rapidly had there been a sense of movement between the two sidesticks.

Whilst the engines were accelerating, the captain applied sufficient back stick to keep the aircraft in the air. He was concerned about the aircraft's performance, particularly during the time required for the engines to accelerate as he judiciously managed the aircraft energy balance, gaining altitude whilst minimising the reduction in aircraft performance. As engine pwer increased, the aircraft nose attitude was adjusted to ensure that the A320 tail would not strike the ground and to ensure that the two aircraft would not collide.

The characteristics of the A320 and the crew behaviour described in the preceding paragraphs highlight the need to address cockpit resource management procedures, communication and design issues in the operation of such advanced technology aircraft. US studies, for example, have suggested that the 'traditional' CRM training may not be appropriate to the demands of the new generation of highly computerised and automated 'glass cockpit' aircraft.

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Peter B. Ladkin, 1999-02-08
Last modification on 1999-06-15
by Michael Blume