Mode of transport
Occurrence ID
AB-2024-001
Brief status
Occurrence date
Report release date
Occurrence category
Aviation occurrence type
Location
Sydney Airport
Injury level
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 the 23 December 2023, a Boeing 737 departed Gold Coast Airport, Queensland, for Sydney, New South Wales on a scheduled passenger flight.

During the approach to runway 16L[1] at Sydney, the aircraft was configured for landing with the flaps set at 30°. At around 800 ft and stable at 146 kt, the crew received an AIRSPEED LOW warning. The crew observed the top of the amber band on the airspeed indicator (ASI) increased from around 124 kt to 151 kt (see the section titled Minimum manoeuvre speed). In response to the varying airspeed indication, the crew conducted a missed approach and actioned the after‑take-off checklist.

At this time, as all the indications had returned to normal, the crew prepared the aircraft for a second approach. However, upon selection of flap 15, the crew observed the amber band’s lower and upper limits extend beyond the normal range on the ASI. The crew then observed the LEADING EDGE FLAP TRANSIT caution light illuminate and confirmed on the overhead panel that the number 1 leading edge flap was indicating that it had not extended. The crew conducted a visual check and determined that the leading edge flaps appeared to be extended.

The crew conducted a second missed approach and notified air traffic control (ATC) of a potential flap issue. ATC then vectored the aircraft to the east of the airport, where the crew kept the aircraft configured at flaps 15 and conducted the appropriate checklist.

As per standard procedure, the crew advised ATC that they would require the longer runway 16R for landing, which was approved. Around the same time, the crew observed the LOW FUEL warning light illuminate, indicating there was 1,800 kg of fuel remaining on the aircraft.

On the base leg of the circuit for runway 16R, the crew observed the fuel had reduced to 1,700 kg. Given that ATC were aware that the aircraft had a technical issue, and was vectoring the aircraft for 16R, the crew assumed the aircraft was already receiving a level of priority and determined that it was unnecessary to notify ATC of the low fuel state. At this point, the crew was satisfied that they would land with their final reserve fuel intact (see the section titled Final reserve fuel).

However, as the aircraft approached the localiser,[2] ATC issued the crew with new vectors away from the airport to allow other aircraft to land before them. This was to reduce the delay to the other aircraft as there was a mandatory requirement to conduct a runway inspection after an aircraft landed with a technical issue.

The crew accepted the vectors but advised ATC that the aircraft only had 1,500 kg of fuel remaining and needed to land soon. As a result, ATC vectored the aircraft back to the localiser and they were cleared to land ahead of the other aircraft. The crew conducted a flap 15 landing without further issue. However, the crew observed that the fuel gauge was indicating 1,000 kg on touchdown, which was below their calculated final reserve fuel (see the section titled Fuel gauges).

Figure 1: Flight path

Figure 1: Flight path

Source: FlightRadar24, annotated by ATSB

The operator’s review of the flight data following the occurrence identified that the aircraft touched down with 1,019 kg of fuel, 35 kg below the retrospective calculation of 1,054 kg final reserve fuel.

The post-flight engineering inspection determined a faulty sensor on the number 1 slat to be the source of the AIRSPEED LOW warning.

Context

Minimum manoeuvre speed

The aircraft’s flight management computer calculates a minimum manoeuvre speed, which provides a margin above the pre-aerodynamic stall[3] buffet speed for the configuration of the aircraft. This speed is indicated by the top of the amber band on the speed tape on the primary flight display (Figure 2).

Figure 2: Sample of B737 primary flight display airspeed indicator

Figure 2: Sample of B737 primary flight display airspeed indicator

Source: Boeing 737NG flight crew training manual, annotated by ATSB

Fuel gauges

The fuel gauge on the primary display indicates fuel levels to the nearest 100 kg, meaning the gauge could be indicating up to 49 kg less than the fuel on board.

Fuel policy

The operator’s fuel policy specified the minimum fuel required for the flight, including the necessary final reserve fuel (See the section titled Final reserve fuel).

In the case of the occurrence flight, the operator advised that the flight was planned in accordance with the operator’s fuel policy, and the captain chose to load an additional 500 kg of fuel prior to departure. Discretionary loading of additional fuel by an aircraft captain is permitted under the operator’s fuel policy.

Final reserve fuel

A turbine-engine aeroplane is required to carry 30 minutes of fuel to allow the aircraft to fly at holding speed, at 1,500 ft above the aerodrome elevation. This must be available at the completion of the flight.

Emergency fuel state

Civil Aviation Safety Regulations Part 121 manual of standards (MOS) 7.08 stated

Where the PIC has calculated that the aircraft will land with less than the final reserve fuel, the flight crew

must declare a situation of ‘emergency fuel’ by broadcasting ‘MAYDAY MAYDAY MAYDAY FUEL’.

As this is a distress message, the aircraft will be given priority to land.

The declaration of a FUEL MAYDAY is an internationally recognised procedure associated with the standards of the International Civil Aviation Organization and designed to assist in the management of aviation safety risks.

The MOS further stated that the pilot in command (PIC) should advise ATC of a minimum fuel state if they are committed to land at an airport and where any changes to the clearance will result in the aircraft landing with less that the final reserve fuel. Significantly however, flight crew should not expect any form of priority handling as a result of declaring minimum fuel.

Safety message

The intent of a FUEL MAYDAY declaration is to alert air traffic control and other airspace users to an aircraft with a low fuel situation, and ensures priority is given to that aircraft.

As in this case, when a situation unfolds where flight crews are required to deviate from the planned flight resulting in a concern that the aircraft may not have the required fuel reserves, flight crews are reminded of the importance of declaring a FUEL MAYDAY.

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 action.


[1] Runway number: the number represents the magnetic heading of the runway. The runway identification may include L, R or C as required for left, right or centre.

[2] The component of an ILS which provides azimuth guidance to a runway. It may be used as part of an ILS or independently.

[3] Aerodynamic stall: occurs when airflow separates from the wing’s upper surface and becomes turbulent. A stall occurs at high angles of attack, typically 16˚ to 18˚, and results in reduced lift.

Aircraft Details
Departure point
Gold Coast, Queensland
Destination
Sydney, New South Wales
Model
737
Sector
Jet
Operation type
Part 121 Air transport operations - larger aeroplanes
Damage
Nil
Manufacturer
The Boeing Company