Interference from a departing aircraft disrupted the ILS signal moments before a landing Singapore Airlines Boeing 777-300ER veered off the runway at Munich.

German investigation authority BFU says the 777 crew was conducting an automatic landing, a decision it describes as “understandable” given the weather conditions – a cloud base of 300ft and visibility of 2km.

The airport was operating under Cat I which did not require the increased aircraft separation necessary during Cat II or Cat III operations.

BFU says the 777 pilots did not tell the approach controller that they intended to perform an automatic landing to runway 08R.

The controller, as a result, allowed a BAE Systems Avro RJ85 to enter the runway and take off while the approaching 777 was 2.1nm from the threshold.

As the 777 crossed the threshold at 50ft the RJ85 was climbing away and passed in front of the localiser antenna, “significantly” interfering with the signal, says the inquiry. The RJ85 was flying relatively low because it had conducted an intersection take-off and had a lower climb rate.

Because the 777 was carrying out an automatic landing, the disturbance to the ILS caused it to bank slowly to the left, up to 3.5°, and the crew responded with an attempted go-around.

The inquiry believes the crew had considered the possibility of ILS interference, and had been prepared to initiate a missed approach, but were nevertheless “confused” by the aircraft’s late deviation.

While the captain called for the go-around flap setting and the take-off/go-around switches were pressed, the aircraft did not respond with go-around thrust. The inquiry found that, at the moment the switches were pressed, the aircraft’s left-hand main gear touched down – and Boeing’s system logic inhibits the go-around mode in such an event.

“This prevented the crew from being able to initiate an automatic go-around,” says BFU. It adds that the captain subsequently decided not to attempt a manual go-around because, by then, the aircraft had switched to ground mode.

“The crew stated that, in their estimate, a go-around procedure initiated manually with an [aircraft] already on the ground would have been much more dangerous than remaining on the ground,” it states.

As the aircraft rolled out along the runway the autopilot remained engaged. It used the rudder and nose-wheel to follow the localiser signal, steering the aircraft to the left.

BFU says the captain applied right rudder to try to keep the aircraft on the runway but, owing to the autopilot’s remaining engaged, the control input remained “ineffective”.

The aircraft veered off the left side of the runway at 123kt some 944m beyond the threshold, rolling for 400m through the grass. As both pilots applied right rudder through the pedals, the autopilot disengaged and the 777 swung to the right by 40°, re-entering the runway about 1,566m from the threshold.

It crossed the centreline of 08R, with a heading of 120°, and veered off the right side of the runway at 71kt before coming to a halt in the grass, 35s after touchdown.

Despite the excursion the aircraft was undamaged, because it had not sunk in the rough ground, and the crew did not regard a slide evacuation as necessary. The passengers disembarked by stairs.

None of the 162 occupants was injured during the incident, which occurred on 1 November 2011.

Source: Cirium Dashboard