Description

On 4 April 2013, during a brief runway closure for a routine daily inspection by three vehicles, a fourth vehicle entered the runway without ATC awareness in normal daylight visibility and was still there, still without ATC awareness and moving along the runway in the direction opposite to aircraft use, when the runway was re-opened and the first landing clearance was issued. The TWR subsequently observed the vehicle on the runway visually but were unable to make contact with it and the aircraft on approach was instructed to go around when just under 1 nm from touchdown.

Investigation

An Investigation was carried out by the AAIB of the Singapore Ministry of Transport.

It was established that a planned 5 minute closure of runway 20R for a daily inspection using three vehicles was also to be used for a fourth vehicle to undertake a surface friction test. The driver of the vehicle to be used was briefed by the Airside Safety Inspection Team (ASIT) supervisor to be ready to begin the task when the runway was closed but had been late departing base.

On the way to the runway, the driver reported attempting to contact the TWR for clearance to enter but receiving only distorted unintelligible replies. At one point, he contacted the team leader by mobile phone to see if the latter had heard the TWR replying to his calls but the reason for and significance of the call was not appreciated by the team leader who simply responded that he had “not heard anything”. Although it was noted by the Investigation that the driver could have asked the team leader to relay his clearance request to the TWR or even called the TWR direct using the same mobile phone, he did neither. Instead, despite not having received the necessary clearance, the friction testing vehicle was driven onto the runway anyway and began the planned test run from the upwind end rather later than had been expected.

Once on the runway, the friction test vehicle passed two of the inspection vehicles, continuing northbound. Whilst the friction test vehicle was still about 1000 metres from the runway threshold, the ASIT leader reported to TWR that his vehicles were clear of the runway and it was reopened, the TWR controller not having noticed any vehicle on the runway visually or – given the good weather conditions observed it on the Advanced Surface Movement Guidance and Control System display. Shortly afterwards, TWR cleared an aircraft to land on runway and at about the same time, one of the runway inspection vehicles called the ASIT leader to advise that the friction test vehicle was still on the runway. The leader was unable to transmit on the TWR frequency because it was busy, but by this time the TWR controller had spotted the vehicle anyway. However, the approaching aircraft, now about 3nm from touchdown, was not notified of the situation and instead the TWR controller made multiple unsuccessful attempts to make contact with the vehicle over a period of 43 seconds before the landing clearance was cancelled and a go around instructed with the aircraft recorded as being just 0.89nm from touchdown.

After further unsuccessful attempts to contact the friction tester vehicle, it reached the end of the runway and vacated. About a minute later, TWR received a call from the vehicle for the first time. It was established that reception of transmissions on the friction test vehicle radio whilst the testing equipment was in use was problematic even with maximum volume set due to the high background noise level created coupled with associated radio interference but that the radio equipment fitted was fully serviceable.

However, it was ascertained that two days earlier, the same friction test vehicle involved had been sent with a different driver to carry out a friction test run at Seletar Aerodrome, located some 8.6nm to the west of Changi. The TWR controller there, where the TWR frequency was 122.9 (compared with the 121.9 being used at Changi at the time of the incident) reported that they had heard ‘disjointed words’ at the time when the friction test vehicle driver had reported transmitting during the incident but had been unable to identify the caller and so had tried - without success - to establish two way contact.

It was concluded that this, coupled with the vehicle drivers admission that he had not checked that the correct frequency was selected before beginning to transmit, almost certainly explained the failure to communicate with the TWR - but not entry to the runway without clearance, since such a clearance was still a requirement even with the runway temporarily closed. In seeking some form of ‘explanation’ for the latter non compliance, the Investigation noted that the driver’s motivation to complete the test may have been strengthened by the knowledge that it was a necessary preparation for planned preventive maintenance work scheduled for two days later which included rubber deposit removal from areas on the runway that were to be identified using the results of the friction test. If the test was not completed this time and, for any reason (such as rain) could not be conducted the following day, then the friction test mission would be a failure of some consequence.

The Investigation made a number of observations on matters related to their findings including that:

• the TWR controllers were supposed to alert an aircraft on final approach of any potentially hazardous circumstances (such as unauthorised traffic on the runway) immediately but “instead of according priority to the approaching aircraft by cancelling the landing clearance and explaining the reason, the controller tried to contact (the vehicle) probably in the hope of getting it (off) the runway in time and thus obviating the need to cancel the landing clearance”. Even when cancelling the landing clearance and giving the instruction to go around, no reason was given.

• it was not established whether any concerted attempt had been made to visually scan the runway from the TWR cabin before declaring the runway re-opened - or to establish how likely it was that, at the range involved, that such a small object, albeit a moving one, would have been readily detectable anyway.

• before clearing the inbound aircraft to land, the controller involved had visually scanned the runway (without spotting the vehicle) but had not checked (and neither was this required in good weather) the A-SMGCS display.

• the five minute inspection period was so short that it was probably unsuitable for the additional friction testing task because the time required to complete it was only just within the planned duration of the closure.

• the single manning of the friction test vehicle where, when on the runway, the driver has to drive at a constant speed in a straight line whilst monitoring the pressure of the water supply tank whilst at the same time looking out for other traffic on the runway and monitoring the radio communication leaves little capacity to respond to any unexpected situations without abandoning the test.

The formally stated Conclusions of the Investigation were that:

• The runway incursion was a result of (the friction test vehicle) entering the runway without Changi Tower’s authorisation. He received distorted radio transmissions from an unidentified source but somehow interpreted them as an authorisation to enter the runway.

• The radio set in (the friction test vehicle) was most likely set to the Seletar Tower frequency of 122.9 MHz instead of the Changi Tower frequency of 121.9 MHZ. The driver was apparently not aware of the frequency setting and thus was not able to communicate with Changi Tower despite his attempts.

A number of Safety Actions were noted as having been taken as a result of the investigated event including:

• the decision to acquire an additional friction test vehicle so that both Changi and Seletar have a dedicated test vehicle fitted with a radio which is pre-selected to the correct TWR frequency.

• the adoption of a new operating procedure whereby all runway maintenance works such as sweeping, friction testing or edge grass cutting will be scheduled in a 15-minute inspection slot rather than a 5 minute one.

• a new requirement for all vehicles permitted to enter the manoeuvring area to be fitted with a Mode-S transponder to enable TWR awareness of their position with more certainty.

One Safety Recommendation was made as a result of the Investigation as follows:

• that the Regulatory Authority consider making it a standard procedure for the Changi Tower to check the A-SMGCS after every runway inspection to ensure that the runway is clear of traffic prior to a landing or take-off, regardless of the visibility conditions.

The Final Report was published on 17 December 2013

Further Reading

• Runway Incursion
• ATC Radio Use by Airside Vehicles
• Advanced Surface Movement Guidance and Control System (A-SMGCS)
• Situational Awareness
• Continuation Bias
• Runway Surface Friction