Select Committee on Science and Technology Fifth Report


Note of visit to British Airways Maintenance, Cardiff

1.  To provide a context for the technical evidence that would be received during the Inquiry, a party from the Sub-Committee visited British Airways Maintenance, Cardiff (BAMC) on 13 April 2000 to:

    (a)  see at first-hand the various elements that control the cabin environment in a typical commercial passenger aircraft; and

    (b)  understand maintenance needs and practices.

2.  The visiting party consisted of Baroness Wilcox (Chairman of the Sub-Committee), Lord Flowers, Lord Jenkin of Roding and Lord McColl of Dulwich, supported by the Sub-Committee's Specialist Adviser (Dr D Michael Davies) and Clerk (Mr Roger Morgan).

3.  The visiting party was welcomed to BAMC by the General Manager, Mr Bruce Hunter. He explained that BAMC serviced aircraft for a number of airlines, including British Airways (of which it was a wholly-owned subsidiary) under broadly similar contractual arrangements with the operators.

4.  Mr Marc White, Technical Services Group Leader, briefed the visiting party on the ventilation arrangements of a Boeing 747-400, the main elements of which were as follows.

    (a)  On the ground, cabin air was provided by a bleed from the compressor stage of an auxiliary power unit (APU) in the tail. The APU also provided power for the air conditioning although, for very hot locations, air conditioning could be supplemented by locally supplied external units.

    (b)  At cruising height, the external air was both too cold and at too low a pressure to be used without treatment. It was compressed in the compression stage of the turbines feeding the jet engines. By compression alone, the air was heated to some 350ºF.

    (c)  This compressed air was ducted to the air conditioning bay between the wings at the bottom of the fuselage. Here, most of it was cooled by external air passing over heat exchanger fins and further, as necessary, by a refrigeration unit. The pressurised and cooled air was then spun through a vortex to remove any condensed water.

    (d)  Suitable pressure was maintained in the cabin by balancing the rate at which air was supplied and vented away. Air from the galleys and toilet cubicles was expelled immediately. Of the remainder, half was allowed to bleed away and half was filtered through high efficiency particulate air (HEPA) filters, as used in hospitals, and mixed with the newly conditioned air for distribution to the various zones in the aircraft cabin. This re-circulation helped boost humidity of the very dry external air to about half the levels normally experienced on the ground in the United Kingdom.

    (e)  The same air was supplied to all zones. The only difference between zones was that the temperature could be controlled individually by the crew. This was done by altering the amount of hot pressurised air fed direct into the circulated air.

    (f)  The air was injected along each outside edge of the cabin ceiling, and extracted along each edge of the cabin floor. Vents directed the air to across the top half of the cabin to the centre. It was then drawn back across the bottom half of the cabin to extractor ducts at floor level. The vents were designed to minimise flow along the cabin.

    (g)  The whole volume of air was exchanged every two or three minutes. As noted, half the replacement was re-circulated air so, while enough fresh air was drawn in to replace the cabin air completely every five minutes or so, the actual replacement was a form of progressive dilution.

    (h)  Increasingly, aircraft did not have individual air nozzles for each passenger. Where fitted, these supplied the same air as otherwise available, but the air felt cooler because of the concentrated movement.

    (i)  All air controls were managed from the flight deck. Normally, this was done automatically although manual control was possible, as might be required in an emergency. For example, it might be desired to flush the cabin with fresh air. Alternatively, the supply of fresh air could be shut off if it were tainted - in which case, oxygen could be supplied to passengers through individual masks.

5.  Mr White also briefly outlined the ventilation arrangements on the newer Boeing 777. These were very similar to the 747 except that incoming air was treated to remove ozone; only fresh air was fed to the flight deck; re-circulated cabin air was passed over an anti-fungal felt; and, by design, there were differences in some of the service intervals.

6.  The visiting party then visited a stripped-down Boeing 747-400 to inspect its systems at first-hand. During this and subsequent discussion, the following points were noted.

    (a)  The HEPA filter-packs seemed very robust, allowing minimal chance of failure. Even so, it appeared that there was no check for any failure during the two year service life. (However, an alarm would sound if a filter were blocked.) Nor were any routine tests made of filters after replacement to assess the job they had done.

    (b)  New filters did not reach peak efficiency until they had been used for a while. However, there were no arrangements for pre-treating filters before installation.

    (c)  Both the maintenance arrangements and qualifications of maintenance personnel were tightly controlled by the regulatory authorities. While part of the maintenance arrangements relied on self-certification, this was covered by strong audit and reporting arrangements that should minimise the possibility of any shortcuts in quality control.

7.  On departure, the Chairman thanked Mr Hunter, Mr White and their colleagues for a most useful and well-arranged visit.

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