| Occurrence Briefs are concise reports that detail the facts surrounding a transport safety occurrence, as received in the initial notification and any follow-up enquiries. They provide an opportunity to share safety messages in the absence of an investigation. |
What happened
On 4 September 2020, the crew of a Diamond Industries DA 40 were on a dual training exercise and conducted a stop-and-go landing at Adelaide, South Australia. After landing, air traffic control (ATC) cleared the aircraft to taxi to reposition for take-off advising there was a Boeing 737 on the holding bay ahead of them. As the DA 40 approached the taxiway adjacent to the holding bay, the instructor noted the grass moving but continued as instructed.
The taxi clearance required the aircraft to make a left turn and as the crew initiated the turn, the aircraft entered the jet blast[1] of the 737 (Figure 1). The instructor was unable to manoeuvre as required with differential braking[2] and made the decision to attempt a turn to the right to exit the jet blast. Upon releasing the brakes and applying right rudder, the pilot reported the DA 40 abruptly turned to the right and the pilot then taxied back towards the runway.
The crew notified the controller they had lost rudder control and ATC responded that the jet was conducting 50 per cent power ground runs.
Figure 1: Location of jet blast
Source: Aerodrome diagram excerpt provided by operator and annotated by the ATSB
Jet blast and effect on controls
Many manufacturers provide information on predicted velocities and safe distances from jet engine exhausts. Figure 2 shows the predicted exhaust gas velocity using breakaway thrust power settings behind a 737-400 similar to the aircraft in the occurrence. It should, however, be noted that breakaway thrust is approximately 35 per cent power under normal circumstances and this aircraft was conducting maintenance ground runs using 50 per cent, meaning the exhaust velocity would be significantly greater than those indicated in this diagram. The DA 40 passed approximately 80 m behind the 737. At breakaway thrust, the DA 40 would be expected to encounter winds in excess of 30 kt, which is beyond the DA 40 maximum demonstrated crosswind limit of 20 kt. This contributed to the handling difficulties experienced while taxiing.
A review by the airside manager of Adelaide Airport found that the 737 maintenance ground runs were conducted in the appropriate location. The controller believed the level of power being used would not affect the DA 40 given the distance from the 737 and therefore there was no requirement to issue a caution to the DA 40 crew.
Figure 2: Predicted exhaust gas velocity for 737-400 aircraft
Source: Boeing annotated by the ATSB
Operator’s investigation
The operator conducted an investigation that included a review of radio transmission transcripts. Their investigation found that there was no advice or warning given to the DA 40 crew by ATC that the 737 was performing ground runs, or that these were being conducted at 50 per cent power. This was despite the controller being aware of the power setting be used. Although the pilot observed the grass moving along the taxiway, they took no action and it was determined that the crew were unfamiliar with the hazards associated with jet blast. The aircraft was exposed to the jet blast for approximately 1 minute.
Safety action
Airservices Australia advised the ATSB that the unit tower supervisor issued a ‘lessons learned’ to raise awareness of the event.
This was the first jet blast occurrence for the operator and as a result of this occurrence, the operator has advised the ATSB that it is taking the following safety actions:
- a review of theoretical training relating to identifying and managing jet blast
- a review of actions to be taken in the event of a jet blast occurrence including initial actions and inspections to be carried out by crew and maintenance personnel.
Safety message
This incident highlights the importance of situational awareness for pilots of smaller aircraft operating around larger aircraft. Jet blast is a hazard at all airports where high performance or transport category aircraft operate. While the risks are generally recognised within the ramp environment, jet blast can be encountered anywhere greater than idle power settings are used.
Avoiding a jet blast hazard requires pilots of light aircraft to be aware of the following:
- the potential danger area behind large jets
- the increased risk potential when aircraft may be about to move or use higher than idle power settings
- being attentive to, and taking cues from, indicators in the operational environment, such as wind socks and grass at the edge of a taxiway.
Notwithstanding the need for pilots’ situational awareness, ATC also needs to be aware of the effects of jet blast on light aircraft. When there is a specific hazard, the requirement for controllers regarding jet blast is detailed in the Manual of Air Traffic Services section 12.1.1.2. Airservices Australia Aeronautical Information Publication stated that ATC should provide a caution to the aircraft. Additionally, taxi clearances should facilitate movement of light aircraft away from jet blast hazards.
About this report
Decisions regarding whether to conduct an investigation, and the scope of an investigation, are based on many factors, including the level of safety benefit likely to be obtained from an investigation. For this occurrence, no investigation has been conducted and the ATSB did not verify the accuracy of the information. A brief description has been written using information supplied in the notification and any follow-up information in order to produce a short summary report, and allow for greater industry awareness of potential safety issues and possible safety actions.
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- Jet blast: the hazard associated with the blast force generated behind a jet engine, especially at high engine power settings when taxiing, before and during take-off, and during engine maintenance activity.
- Differential braking: The use of independent braking systems installed on the left and right wheels of an aircraft to assist in steering.