Aviation safety investigations & reports

Boeing Co 737-800, VH-VXD

Investigation number:
Status: Completed
Investigation completed


The co-pilot of a Boeing 737-800 (B737) en route from Perth to Brisbane on air route T91 reported maintaining flight level (FL) 390. He later reported maintaining FL370 at APOMA, located 140 NM north-northeast of Broken Hill, at 0501 EST. The Bourke sector controller in the Melbourne Air Traffic Control Centre requested the co-pilot to confirm the aircraft's level. The co-pilot replied that the aircraft was at FL370. A Boeing 747 (B747) maintaining FL370 was on a crossing route and estimated APOMA at 0503, two minutes after the B737. To be separated, aircraft at the same level on intersecting tracks required 15 minutes between their respective intersection estimates. The controller issued traffic information to the co-pilot of the B737 and to the crew of the B747.

At a subsequent position report the pilot in command (PIC) of the B737 overheard the co-pilot report the aircraft's level as FL370 and corrected the error. Later analysis of the flight data recorder of the B737 confirmed that the aircraft had maintained FL390. There was no infringement of separation standards.

The B737 operator had an approved procedure for the `controlled rest' of flight crew members while remaining on the flight deck. Controlled rest was recognised by the operator as `an effective method of improving levels of crew alertness for critical phases of flight'. There were a number of guidelines concerning the use of the procedure, including:

  1. It was not to be used on sectors of less than two hours duration.
  2. It was only to be used during the cruise phase of flight.
  3. Periods of controlled rest were not to exceed 30 minutes per crew member per sector.
  4. An additional 10 minutes was required after the period of controlled rest before a crew member resumed flight deck duties.

At 0213, the B737 departed Perth and was climbed initially to FL370. The flight plan advised an intention to change level to FL390 by BEZZA, a position about 340 NM west of Leigh Creek. At about 0405, the co-pilot requested and was approved by the PIC to take a controlled rest. At 0416, the PIC requested and was approved by the Melbourne Centre controller for the aircraft to climb to FL390. At 0421, the PIC reported maintaining FL390 to the Melbourne Centre controller. At about 0440, the co-pilot ceased the controlled rest and was briefed by the PIC in the next five minutes before assuming control of the aircraft. The briefing included the level of the aircraft. The PIC then commenced a period of controlled rest until about 0520. On initial contact with the Bourke sector controller at 0458, the co-pilot reported that the aircraft was maintaining FL390.

The roster for the 48-hour period preceding the incident required the crew to commence work at about 1900 two days before the incident and to operate a flight from Brisbane to Perth, arriving in Perth at approximately 0120. They spent the remainder of that day at their leisure prior to departing Perth for Brisbane on the incident flight at 0213 the following morning. That departure time required the crew to report for duty at approximately 0100.

On the morning of the Brisbane to Perth flight the co-pilot awoke at 0530 and had a 30-minute nap during that day. The co-pilot slept for approximately 6.5 hours after arriving in Perth, had a 90-minute nap late that afternoon and 30 minutes of controlled rest during the flight to Brisbane. Those periods of sleep accumulated to about nine hours during the 48-hour period. The co-pilot later reported that he felt rested prior to commencing the Perth to Brisbane flight.

Fatigue is recognised as a primary cause of transport accidents throughout the world as a result of reduced or impaired mental and/or physical performance following inadequate rest.

One component of fatigue relevant to the transport industry is sleep inertia. That phenomenon refers to the period of mental dullness or sluggishness immediately after awakening. During a period of sleep inertia people demonstrate the outward signs of being awake but are not mentally awake. Research suggests that it may take approximately 30 minutes after awakening for the effects of sleep inertia to fully dissipate.

The co-pilot was newly rated on the B737-800 and reported that he had found scanning the altitude indicator in that aircraft series took longer and required additional conscious effort. The co-pilot reported that when providing the aircraft's level, in response to the Bourke sector controller's query, he had relied on his memory of the information in the position report rather than re-checking the altitude indicator on the flight instrument display.

The Bourke sector controller twice requested confirmation of the flight level from the co-pilot of the B737 and was advised on both occasions that the level was FL370. At that stage, the controller passed traffic information to the co-pilot about the B747 at FL370 on the crossing route. The controller also passed traffic information on the B737 to the pilot of the B747. The co-pilot of the B737 later reported that at that time he was unsure why the controller had issued the traffic information as he could see the B747 displayed on the B737's Traffic Alert and Collision Avoidance System (TCAS) behind and 2,000 ft below his aircraft.

The Manual of Air Traffic Services (MATS) section covering safety alerts (Section 5.1.13) included the following:

'A safety alert shall be issued to an aircraft when a controller is aware the aircraft is in a situation which is considered to place it in unsafe proximity to terrain, obstructions, or other aircraft. The controller must remain vigilant for the development of such situations and issue a safety alert when the situation is recognised.

Conditions such as workload, traffic volume, the quality/limitations of the radar system, and the available lead time to react are factors in determining whether it is reasonable for the controller to observe and recognise such situations.

The issuance of a safety alert is a first priority.

When a controller is aware that an aircraft is in unsafe proximity to another aircraft, a safety alert shall be issued as follows:

"(Callsign) TRAFFIC ALERT (position of traffic if time permits), [SUGGEST] TURN LEFT / RIGHT (specific heading, if appropriate), and / or [SUGGEST] CLIMB / DESCEND (specific altitude if appropriate), IMMEDIATELY".

When a safety alert is directed to traffic not receiving a separation service, advice to turn or change level shall be prefixed with the word SUGGEST'.


The co-pilot had less than nine hours of interrupted sleep during the 48 hours preceding the incident. That small period of sleep suggests that the co-pilot may have been fatigued at the time of the incident despite feeling adequately rested. Fatigue may lead to impaired physical and mental performance in people and may explain why, when the controller requested confirmation of the aircraft's level, the co-pilot relied on his memory rather than performing the more demanding task of confirming the aircraft's level by looking at the altitude indicator.

Within a short time of waking from the period of controlled rest, the co-pilot had received a handover briefing and assumed control of the aircraft. Despite reporting the correct level (FL390) to the Bourke sector controller during a change of radio frequency, the co-pilot subsequently reported an incorrect level (FL370) a few minutes later with the APOMA position report.

It is likely that the co-pilot was suffering from the cumulative effects of fatigue and sleep inertia that resulted in the incorrect level being passed with the position report. The occurrence highlights that an understanding of fatigue and how to manage it are important defences to a human limitation.

Despite the aircraft being at flight levels that provided 2,000 ft vertical separation, the Bourke sector controller's provision of mutual traffic information to both the co-pilot and the pilot of the B747 was warranted. At that stage, the pilot reported information provided to the controller indicated that a separation standard was not being applied to the aircraft. The traffic information would have assisted both crews to assess the potential for conflict and would have provided a basis for pilot-initiated avoiding action. Under the circumstances, the provision by the controller of a safety alert that would have included a recommended action to either turn or to climb/descend, as well as the provision of traffic information, may have been a better option. That action would have ensured that the pilots clearly understood that the aircraft were in close proximity and at the same level, based on the reports provided to the controller.

Safety Action

Local safety action

The B737 operator amended controlled rest procedures to require both crew members to be on duty when a change in level was conducted.

ATSB safety action

The ATSB issued Safety Advisory Notice (SAN) 20010244 to the aviation industry on 2 May 2002. That notice stated:

The Australian Transport Safety Bureau alerts all operators in the transport industry, particularly those involved in extended-hours operations, to the possibility of crew members suffering sleep inertia and suggests that operators take steps to mitigate the effects of sleep inertia. The steps should not include subjecting employees to sleep deprivation.

The ATSB issued SAN20010245 to the Civil Aviation Safety Authority on 15 April 2002. That notice stated:

The Australian Transport Safety Bureau suggests that the Civil Aviation Safety Authority alert all aviation industry operators to the possibility of sleep inertia impairing performance, particularly that of flight and maintenance crews.

The ATSB also issued SAN20020035 to the Civil Aviation Safety Authority on 15 April 2002. That notice stated:

The Australian Transport Safety Bureau suggests that the Civil Aviation Safety Authority ensure that operators have strategies in place to mitigate the effects of sleep inertia as part of their fatigue management systems.

General details
Date: 08 August 2002   Investigation status: Completed  
Time: 0500 hours EST    
Location   (show map): APOMA, (IFR)    
State: New South Wales   Occurrence type: Air-ground-air  
Release date: 03 December 2002   Occurrence category: Incident  
Report status: Final   Highest injury level: None  

Aircraft details

Aircraft details
Aircraft manufacturer The Boeing Company  
Aircraft model 737  
Aircraft registration VH-VXD  
Type of operation Air Transport High Capacity  
Damage to aircraft Nil  
Departure point Perth, WA  
Departure time 0213 hours EST  
Destination Brisbane, QLD  
Last update 13 May 2014