Shortly after commencing descent from flight level (FL) 310, the pilot of the Cessna Conquest noticed that the cabin altitude indication was rising. When passing through FL270, the cabin rate of climb rose to 3,000 ft/min with an accompanying decrease in cabin differential pressure and both master warning and "ALT" annunciators illuminating. When the annunciator lights illuminated, the pilot observed that the cabin oxygen masks had failed to auto-deploy as required. As the patients were already on oxygen and the flight nurse had donned a spare mask, he did not deploy them manually. The pilot donned his own oxygen mask and commenced a rapid descent to 10,000 ft.
The pilot reported that the cabin altitude indication increased to a peak of 17,000 ft before decreasing during the descent. A subsequent maintenance inspection found water contamination present in the cabin door seal pressurisation solenoid valve and it was suspected that this water froze, preventing the correct operation of the valve.
The failure of the oxygen mask auto-deployment system was also investigated and the wiring to the barometric activation switch was found to have a high resistance. This wiring was replaced. As a precaution, the wiring to the barometric switch that activated the cabin altitude annunciator warning light was also replaced. During these rectifications, the maintenance personnel discovered that there was no procedure published by the manufacturer to correctly adjust and test either barometric switch set-point after maintenance.
Manufacturers' maintenance manual procedures
A sampling of maintenance manual procedures for aircraft types, representative of the current Australian fleet and manufactured in countries such as the United States, United Kingdom and Switzerland was carried out. This sampling concluded that the maintenance deficiency for test and return to service of the oxygen deployment and cabin altitude alert barometric switches, found in the occurrence aircraft maintenance procedures, was also apparent in all but one of the other aircraft type manufacturer's maintenance procedures.
The regulations currently in force in Australia state that: `CASA must issue a type acceptance certificate for an aircraft manufactured in a foreign country, without making the type certificate subject to any conditions, if a foreign type certificate or equivalent document issued by the NAA of a recognised country is in force for aircraft of that type'. CASR 21.183 deals with the issue of standard certificates of airworthiness and does not state any requirement specific to these altitude warning and oxygen deployment systems, other than that the general manufacture and modification status must conform to the production certificate or type certificate of an NAA or approved modifications to those standards.
CASA advised that, although CAO 108.26 was still in force, it was not actively applied by CASA during the Australian airworthiness certification process. CASA had introduced new legislation under CAR 22A and CAR 24 in 1990, later superseded by CASR 21.29A and CASR 21.183 respectively, that facilitated the entry to Australia of new types and models without having to comply with Australian design standards. This legislation abandoned the practice of applying Australian design standards to aircraft manufactured outside Australia and certificated in any of five major aviation countries. The Federal Aviation Administration (FAA), being the National Aviation Authority (NAA) of the United States and one of these recognised countries, has also promulgated amendments to FAR Part 25 and FAR Part 23 at amendment 23-17 that require cabin altitude warnings to trigger at 10,000 ft. CASA stated that the latest Australian legislation, in combination with the FAA Regulations, makes the requirements of CAO 108.26 Para 3.1 redundant.
On 12 July 2002, in a further response to recommendation R20000289, CASA recognised that there was scope for confusion and advised that it was considering amending CAO 108.26 to delete all requirements except the operational requirements imposed by CAO 20.4 sub-section 3 and the specification for protective breathing equipment imposed by CAO 20.4 sub-section 10. The response also stated that if CAO 108.26 is amended, the background to the amendment will be explained in an article in `Flight Safety Australia'.
|Date:||31 January 2002||Investigation status:||Completed|
|Location:||222km NW Perth, VOR|
|State:||Western Australia||Occurrence type:||Air/pressurisation|
|Release date:||29 October 2002||Occurrence class:||Technical|
|Report status:||Final||Occurrence category:||Serious Incident|
|Highest injury level:||None|
|Aircraft manufacturer||Cessna Aircraft Company|
|Type of operation||Aerial Work|
|Damage to aircraft||Nil|
|Departure point||Karratha WA|