TUI B738 at Manchester on Oct 23rd 2023, both bleed air systems inadvertently off

Last Update: November 14, 2024 / 20:56:12 GMT/Zulu time

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Incident Facts

Date of incident
Oct 23, 2023

Classification
Incident

Airline
TUI Airways

Flight number
BY-2258

Departure
Manchester, United Kingdom

Destination
Kos, Greece

Aircraft Registration
G-TAWD

Aircraft Type
Boeing 737-800

ICAO Type Designator
B738

A TUI Airways Boeing 737-800, registration G-TAWD performing flight BY-2258 from Manchester,EN (UK) to Kos (Greece) with 187 passengers and 6 crew, was climbing through FL130 out of Manchester when the crew received a "CABIN ALTITUDE" warning and discovered that both bleed air systems had been inadvertently left off after departure. The crew turned both systems on and continued the climb, however, at FL200 the crew received "PACK" caution and stopped the climb again. The crew consulted with maintenance and decided to return to Manchester where the aircraft landed without further incident.

The AAIB released their bulletin concluding the probable causes of the serious incident were:

The aircraft departed with the engine bleed air system off because the switches had been incorrectly left off following maintenance activity and had not been turned on during pre-flight procedures. The after-takeoff checklist is designed to trap the latter omission, but the incorrect switch selection went undetected by the crew. The aircraft failed to pressurise, but the crew did not complete the prescribed QRH drills in response to a cabin altitude warning, which remained illuminated for 43 minutes.

The AAIB summarized the sequence of events:

The taxi out and the departure from Manchester were uneventful. The commander was PF and recalled he manually flew the aircraft until the after-takeoff checks were completed. The after-takeoff checklist, carried out by the PM on a challenge and response basis, includes a check that the bleed air switches are selected on. Both pilots recall completing this checklist and were sure that the bleed switch positions were visually verified as being on.

The commander recalled that, with the aircraft passing approximately 15,000 ft amsl, the cabin altitude warning light illuminated accompanied by the associated warning horn. In his report the commander stated that ‘It was quickly noticed that both engine bleeds were off, these were placed straight back on and the problem was resolved.’ The commander then stated ‘The aircraft was levelled off and the QRH1
was actioned. Once this was completed, shortly afterwards the right pack failed.’ Neither pilot actioned the QRH Cabin Altitude Checklist which contains memory items, including the immediate use of oxygen masks.

The commander recalled noticing that both engine bleed switches were selected to off and that he directed the co-pilot to switch them on. Both pilots recalled that the bleed switches were selected on before they requested a level off at FL150 from ATC. Once the aircraft was level the commander recalled that memory items from the checklist for cabin altitude warning should have been completed. However, as the bleed systems were now on and believing the situation to be under control, he decided that the memory items, including the use of oxygen masks, were disproportionate to the situation.

Once level the commander checked the cabin altitude indication on the overhead panel and recalled it was approximately 2,000 ft and believed this was achieved “within a couple of minutes” of the engine bleed switches being selected on. He then decided that the crew should read through the checklist to ensure there were no actions he felt should be done or any other information they should be aware of. Satisfied that the situation was under control and that the cabin altitude was below 3,000 ft, the crew felt safe to continue the flight and a further climb was requested from ATC.

During the climb the master caution illuminated and drew the attention of the crew to a pack caution on the overhead panel indicating a fault in the right air conditioning pack. The crew requested a level off at FL200 from ATC and actioned the QRH checklist for a right pack caution. They were not able to recover the right air conditioning pack and, aware this would impose some limitations on the return flight, the commander decided to discuss the situation with the operator’s maintenance control (Maintrol). After consulting Maintrol it was agreed that the aircraft should return to Manchester. As the aircraft was above maximum landing weight the crew planned to enter a hold with the landing gear extended to burn fuel and reduce weight. The senior cabin crew member was brought to the flight deck for a briefing and the situation was explained to the passengers over the public address system. Once the aircraft weight had been satisfactorily reduced the aircraft made a normal approach to Runway 05 at Manchester.

The AAIB analysed:

Maintenance work was carried out during the night before this flight during which the engine bleed air switches were selected off. The engineer carrying out the maintenance believed that the switches had been returned to the on position after the work had been completed, but it appears that they were left off. The engineer assigned to the aircraft’s departure did not detect the incorrect switch position.

During the pre-start procedures by the flight crew, it was not identified that the engine bleed switches were set off nor was this oversight detected in the after-takeoff checks. The crew did not use the prescribed memory items in response to a cabin altitude warning and may have misinterpreted the cabin altitude gauge readings after the engine bleed switches were selected to on.

The departure was early in the morning and the commander’s rest had been disturbed by his mobile phone and as a result he had slept for only three hours in the night preceding the duty. Additionally, his workload over recent weeks had been higher than average for the operator and he had been exposed to a number of potentially disruptive overtime duties. It is possible, therefore, that fatigue was a factor in the commander’s decision making. The co-pilot had a lower fatigue risk than the commander but their journey to work had been disrupted and they had arrived late at the aircraft. Although the co-pilot recalled that they did not feel under pressure, they were trying to expedite the departure. It is possible, therefore, that in completing their procedures in an expeditious manner they were more vulnerable to seeing what they expected to see. As both bleed switches were in the same position, off, it is possible the co-pilot perceived them to be on, since that was their expectation.

The engine bleed switches are not mentioned again until the after-takeoff checklist so there was no further procedural opportunity to identify the incorrect switch positions until the aircraft was airborne. After takeoff the PF calls for the after-takeoff checks, which are completed by the PM using challenge and response techniques. It appeared likely that during these checks the co-pilot again saw what they expected to see and so did not notice that the engine bleed switches were still selected off. The switches are on the opposite side of the cockpit to the commander, reducing the likelihood that he would notice that they were in the wrong position.

The aircraft continued to climb with the bleed air systems off and so the cabin failed to pressurise. The cabin altitude warning was triggered with the aircraft at approximately FL130. The aircraft QRH specifies memory items in response to this warning, the first of which is to don oxygen masks and establish crew communications. The crew did not conduct the memory items but in response to the warning the commander noticed the engine bleed switches were off and directed them to be selected on. The crew recalled that this was done very quickly, though in fact 73 seconds elapsed between the warning and the bleeds being selected on, during which the aircraft was levelled at FL150. Although the memory items were not carried out, the pilots were perhaps pre-disposed to check the bleed switches due to the entry in the aircraft tech log, related to air systems, which they had noted earlier. The pilots may have suffered a startle effect at the initial warning, but simple memory items are included in checklists to support crew decision making in crisis and support pilots by creating a structure for them to follow.

Crew training is generally focussed on rapid decompression at high altitudes. This event occurred at a much lower level with much less grave symptoms and so may have subconsciously conditioned the degree of urgency attributed to the response by the crew.

With the bleed switches selected on, the crew assumed that the aircraft systems would correct the cabin altitude and that the rest of the memory items beyond donning oxygen masks would have been disproportionate. However, donning the oxygen masks as an immediate action would have given both pilots immediate protection from any hypoxia risk and allowed them to clarify the situation with the highest risk removed. The crew’s expectation was that selecting bleeds on would have resolved the problem.

The commander’s recollection was that the cabin altitude was 2,000 ft within “a couple of minutes”. The aircraft was level at FL150 for three minutes and 30 seconds before recommencing the climb to FL280. The commander recalled seeing the needle on the cabin pressure gauge as being between the one and two o’clock positions. If the longer needle on the gauge was in that position it would indicate a cabin pressure differential of between one and two psi, suggesting a cabin altitude that was still high. Confirmation bias may have been a factor for the commander as his expectation was that turning on the bleed air system would have rapidly resolved the problem. However, the aircraft pressurisation system would have required over 10 minutes to achieve a cabin altitude of 2,000 ft.

Certain that the problem had been resolved the crew continued the climb and neither pilot recalled the red cabin altitude warning light as being on. During the climb the right pack caution illuminated. The crew stopped the climb at FL200 and actioned the QRH for the pack caution, but this did not recover the pack and so the crew sought advice from the operator’s Maintrol. Dispatching for the return leg to Manchester with a pack inoperative would have imposed limitations on the altitude to which the aircraft could climb and would have incurred significant performance penalties, so the decision was taken to return to Manchester. No technical cause for the pack fault was subsequently found but a likely cause is that the system was placed under a high load by the crew selecting low temperatures for the air conditioning, although neither pilot could recall what temperatures were set on the aircraft.

The recorded data indicated that the cabin altitude warning remained on for 43 minutes. During that time the associated red warning light should have been illuminated, though neither pilot recalls seeing it. As the passenger oxygen masks did not deploy, the cabin altitude did not exceed 14,000 ft but, nevertheless, it was likely that the crew and passengers were exposed to a progressive hypoxia risk. While any loss of consciousness was highly unlikely, a negative impact on the ability of the crew to process information and make decisions was probable.

The climb to cruising altitude was interrupted by the pack caution. Had the aircraft continued to climb, the passenger oxygen system would have deployed at 14,000 ft cabin attitude, and at 15,800 ft the auto fail caution on the pressurisation would have triggered a further master caution, and each of these events would have been a further opportunity to re-assess the situation. With one pack operational, the aircraft would have pressurised at a rate of climb below that of the aircraft, leaving a risk of progressive hypoxia that, coupled with fatigue, could have reduced the crew’s ability to respond appropriately to these indications.
Incident Facts

Date of incident
Oct 23, 2023

Classification
Incident

Airline
TUI Airways

Flight number
BY-2258

Departure
Manchester, United Kingdom

Destination
Kos, Greece

Aircraft Registration
G-TAWD

Aircraft Type
Boeing 737-800

ICAO Type Designator
B738

This article is published under license from Avherald.com. © of text by Avherald.com.
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