Recommendation R20000288

Recommendation issued to: Civil Aviation Safety Authority

Recommendation details
Output No: R20000288
Date issued: 17 December 2000
Safety action status:
Background: Why this Recommendation was developed

Output text

The ATSB therefore recommends that CASA mandate the fitment of aural warnings to operate in conjunction with the cabin altitude alert warning systems on all Beechcraft Super King Air and other applicable aircraft.

Initial response
Date issued: 02 February 2001
Response from: Civil Aviation Safety Authority
Action status: Monitor
Response text:

The Civil Aviation Safety Authority accepts this recommendation and will move to prepare a regulatory amendment to make it mandatory for pressurised aircraft to have aural cabin altitude alert warning systems. This amendment will follow the normal regulatory development process which, in the first instance, will lead to the circulation of a Discussion Paper. It is anticipated that the paper will be released this month.

Further correspondence
Date issued: 10 February 2001
Response from: Civil Aviation Safety Authority
Response status: Monitor
Response text:

I refer to my letter to you of 2 February 2001 in response to your recommendation R20000288.
I would like to clarify CASA's position which is that audible alarms be made mandatory in all turbine powered pressurised aircraft. This is presented as the preferred option in the discussion paper which canvases industry views on the alternatives; these include extending the requirement to all pressurised aircraft or maintaining the current rule.

A copy of the discussion paper will be forwarded to you shortly.

Further correspondence
Date issued: 19 April 2002
Response from: Civil Aviation Safety Authority
Response status: Monitor
Response text:

CASA is moving quickly to publish a Notice of Proposed Rule Making (NPRM) on the above subject. Publication date is intended to be before the end of April.

The objective of the Airworthiness Directive is to provide a minimum acceptable standard for an aural warning system and to fulfil ATSB Recommendation No. R20000288.

Further correspondence
Date issued: 26 March 2003
Response from: Civil Aviation Safety Authority
Response status: Monitor
Response text:

I am writing to advise you of the Civil Aviation Safety Authority's (CASA's) final determination with regards to the Australian Transport Safety Bureau's Recommendation 20000288 that the Authority mandate the fitment of aural warnings to operate in conjunction with the cabin altitude alert warning systems. As you would be aware, the Deputy Prime Minister and Minister for Transport and Regional Services has taken a close interest in this recommendation for some time and has been pressing for a resolution of this matter as soon as possible.

In December 2000, the ATSB issued Recommendation 20000288, stating that `The ATSB therefore recommends that CASA mandate the fitment of aural warnings to operate in conjunction with the cabin altitude alert warning systems on all Beechcraft Super King Air and other applicable aircraft'. I n February 2001, CASA advised the ATSB that the Authority had accepted this recommendation. In its response, CASA advised that the Authority had agreed to prepare a regulatory amendment to make it mandatory for pressurised aircraft to have aural cabin altitude alert warning systems.

To progress this regulatory amendment, CASA issued a Discussion Paper (DP) and a Notice of Proposed Rule Making (NPRM) in order to consult with members of the aviation industry. As CASA advised the Bureau in April 2002, the objective of a proposed AD to follow the NPRM was to provide a minimum acceptable standard for an aural warning system and to fulfil R20000288. While the responses received to both the DP and the NPRM included some support of the proposal, key responses such as those from the Regional Aviation Association of Australia and the Australian Federation of Air Pilots did not provide favourable support.

The comments and subsequent information provided to CASA have caused the Authority to carefully re-consider both the cost-benefit analysis of the proposal, and its acceptance of Recommendation 20000288.

A number of respondents commented that R20000288 was based solely on one incident (VH-OYA), and that that incident involved a newly endorsed Royal Australian Air Force (RAAF) pilot on his first flight as pilot in command of a Beech 200. The RAAF subsequently reported to the ATSB a second pressurisation incident, which occurred on 12 December 1999, involving VHOYA.
Some respondents objected to CASA's proposal on the grounds that the incident involving VH-OYA was not typical of civil operations, especially commercial operations with a multiple-crew. Many respondents explained that the incident involving VH-OYA points to the risk associated with inadequate training rather than a need for additional hardware. In short, comments received by CASA indicated that the nature of the flight conducted by VH-OYA was dramatically different to operations conducted by those potentially affected by the recommendation.

CASA's cost-benefit analysis for the mandatory fitment of aural warnings was based on an assumed fatality rate in Australia caused by unrecognised depressurisation. Several respondents highlighted the fact that if this assumed fatality rate was extrapolated to the United States of America the fatality rate would be so high that the Federal Aviation Administration would respond immediately, with CASA therefore only having to harmonise with the resulting requirement.

CASA believes that if a cost-benefit analysis were to be based on worldwide historical data for accidents caused by, or possibly caused by, cabin depressurisation the result would not support mandatory installation of aural warnings. This would certainly be true of modern transport category aeroplanes whose warning systems usually cannot be modified by the addition of simple, low-cost aural warning systems.

CASA has been unable to find any support for the proposal to mandate the fitment of aural warnings amongst foreign regulators responsible for certification of affected aircraft, nor from the manufacturers of these aircraft. However, it is noted that some manufacturers are fitting aural warning devices in newer aircraft types such as the Raytheon King Air B300.

For over a decade CASA, and its predecessors, have followed a policy of international harmonization, in that uniquely Australian requirements will not be introduced unless the requirement can be justified on safety grounds. This policy accords with the Single Worldwide Certification Code and the international desire to not devalue aircraft by local modification.

I understand that CASA has recently been advised by De Havilland Canada that the development of a system capable of satisfying the proposal for the Dash 8 aircraft is estimated to cost $317,650 (CAD). This cost could be shared among the operators requiring the retrofit but operators would be responsible for procuring and installing the required parts, with subsequent flight-testing of at least two hours.

The fatal accident involving VH-SKC in May 2000, focussed public attention on the possibility that absence of an aural warning in a pressurised aircraft may be a serious deficiency.

CASA notes that the Western Australian State Coroner, Mr Alastair Hope, was unable in his findings regarding VH-SKC to confidently attribute the accident to unrecognised depressurisation. Also, the ATSB's final report concluded that 'due to the limited evidence available, it was not possible to draw definitive conclusions as to the factors leading to the incapacitation of the pilot and occupants of VH-SKC.'

The Coroner subsequently concluded that `While it is possible that the occupants of the aircraft died as a result of hypobaric hypoxia, I cannot exclude the possibility that some unknown and unidentified toxic fumes caused their incapacity and death ...'. Nevertheless, Coroner Hope recommended that `an audible warning, to operate in conjunction with the cabin altitude alert system, be fitted to pressurized aircraft, such aural alert to be triggered by a separate barometric switch from the visual alert system and at a different cabin pressure altitude.'

In the absence of clear proof to the contrary, CASA is not convinced that unrecognised depressurisation was the cause of the accident involving VH-SKC and believes that this accident therefore does not of itself justify the mandatory fitment of audible warning devices.

The key issue is, of course, how do we proceed from here. CASA now considers that it is not appropriate to mandate the fitment of aural warning devices. While it would appear that fitment could be achieved for some aircraft types at a reasonable cost, manufacturers have advised us it would be prohibitive for other types.

In view of the recent availability of low cost devices that may provide safety benefits to some aircraft types, CASA proposes to individually write to owners of affected aircraft types drawing their attention to the devices and recommending their fitment be considered. While these devices would be fitted on a 'no hazard, no interference' basis and on the understanding that there is no standard that would guarantee their functionality in the event of an emergency, CASA intends to promote the potential benefit of these systems, particularly for operators of the Beech 200 and other pressurised single-pilot aeroplanes.

In light of the above, CASA's revised response to Recommendation R20000288 is as follows:

CASA acknowledges the potential safety benefit in installing an aural warning system in Beech 200 and other single-pilot pressurised aeroplanes. However CASA does not consider the potential safety benefits sufficient to warrant mandating the fitment of aural warnings. CASA notes no manufacturer or certifying Authority in the world requires mandatory fitment of such aural warnings and the initiative is opposed by key industry stakeholders in Australia.

CASA will however continue to strongly recommend aural cabin pressure warning devices and will write to the registration holders of all single-pilot pressurised aeroplanes, including the Beech 200, advising them of the availability of low-cost aural warning systems and their benefit in relation to uncommanded cabin depressurisation events.

Through continued safety education programmes, CASA will also continue to inform pilots about hypoxia, the importance of pressurisation management and the appropriate response to an uncommanded depressurisation.

In view of CASA's commitment to consult with the aviation community on all its proposed legislation I believe this is a sound and defensible position for CASA to take in response to R20000288 and meets the intent of what the ATSB was trying to achieve in its recommendation and will serve to improve future aviation safety.

I trust that this response will allow you to close this recommendation.

ATSB response:

Thank you for your letter BE02/72 of 25 March 2003, responding to ATSB Recommendation 20000288.

As you know the ATSB formulates recommendations to enhance safety, where necessary through implementation of additional safety defences. In this case, the ATSB considered that regulatory action would greatly assist in the implementation of the safety enhancement. This had apparently been endorsed by CASA since February 2001 when the Authority first advised that it had accepted the recommendation. The ATSB is disappointed that CASA has now changed its position with respect to the recommendation, but notes the Authority's detailed explanation for doing so.

The Bureau believes the actions CASA propose in its response represent a significant positive step towards improving future safety. The Bureau will MONITOR the Authority's proposed safety action and the "take up rate" of installation before formally assessing the recommendation for closure.

Further correspondence
Date issued: 27 May 2003
Response from: Civil Aviation Safety Authority
Response status: Monitor
Response text:

On 27 May 2003, the Civil Aviation Safety Authority advised the ATSB that the authority had issued the following letter dated 26 May 2003 to certificate of registration holders for pressurised aircraft:


Aural cabin pressure warning system

In 2000 the Australian Air Transport Safety Bureau (ATSB) recommended that the Civil Aviation Safety Authority (CASA) make it mandatory for all Beech 200 and other applicable aircraft to be equipped with an aural warning of low cabin pressure. This recommendation' was based on an incident in which a Beech 200 aircraft suffered an uncommanded cabin depressurisation. The visual warning system activated but the pilot's performance was degraded to the point that he did not respond correctly. In that incident an accident was avoided by the actions of a passenger.

On 4 September 2000 a Beech 200 aircraft took off from Perth and flew for five hours before crashing in Queensland. There were no survivors. The ATSB investigation concluded that while there are several possible reasons for the pilot and passengers being incapacitated, this was probably a result of hypobaric hypoxia due to the aircraft being fully or partially unpressurised and the occupants not receiving supplemental oxygen.

Cabin altitude significantly above 10,000 feet leads to hypoxia, a condition of impaired human performance caused by inadequate oxygen in the blood. One of the early symptoms of hypoxia is impairment of vision and deterioration of situational awareness. There is general consensus that when suffering hypoxia a pilot's ability to react to an aural warning is impaired less rapidly than the ability to react to a visual warning. In an uncommanded cabin depressurisation the occupants' physical and mental functions will suffer progressive impairment and early recognition and rectification of the situation is vital.

Aural warning recommended

CASA responded to the ATSB recommendation by publishing a Discussion Paper in 2001, and a Notice of Proposed Rule Making (NPRM) in 2002. Both these consultation documents proposed that CASA would issue an Airworthiness Directive to mandate an aural cabin pressure warning in turbine engine pressurised aircraft. The response of the aviation community to the NPRM was mixed, but the most persuasive responses were opposed to CASA's proposed action.
As a result, CASA decided not to mandate aural cabin pressure warnings in any aircraft at this time. In particular, it was indicated that the cost of mandatory fitting of aural warnings could be prohibitive for some aircraft.

For many years CASA has however, strongly recommended that the cabin pressure warning system in pressurised aircraft should include an aural warning in addition to the visual warning. Unfortunately, low-cost aural warning systems suitable for installation in pressurised aircraft have not been available until recently. At least two low-cost aural warning systems are now readily available in Australia. CASA again strongly recommends that, if your pressurised aircraft does not have an aural cabin pressure warning, you seriously investigate installation of such a warning system. The benefit to your pilots and passengers lies in the reduction in risk of an uncommanded depressurisation leading to an incident or fatal accident. The benefit is much greater than the cost of purchase and installation of one of these low-cost systems.

Safety measure

An aural cabin pressure warning system is a positive safety measure for all pressurised aircraft. Clearly, the benefit could be substantial in an aircraft operated by a single pilot. The benefit could also be substantial in an aircraft that routinely operates at high altitude, say above 18,000 feet. If your pressurised aircraft does not have aural cabin pressure warning and is routinely operated above 18,000 feet by a single pilot CASA strongly recommends you give prompt consideration to installation of an aural cabin pressure warning system.

Two Australian manufacturers have approached CASA to demonstrate low-cost aural cabin pressure warning systems which are now in production:

Aironautical Electronics Corporation
Phone: 08 8362 4142
Fax: 08 8362 4194
See advert in Flight Safety Australia

Electric Force Measurement
Phone: 03 955& 5741
Fax: 03 9558 5741

While CASA has not made it mandatory for pressurised aircraft to be equipped with aural cabin pressure warning systems at this stage, we strongly encourage each operator of a pressurised aircraft to give serious consideration to the benefit of voluntarily installing such a system if the aircraft presently has only a visual warning system.

Further correspondence
Date issued: 23 April 2004
Response from: Civil Aviation Safety Authority
Response status: Monitor
Response text:

The following response was received from the Civil Aviation Safety Authority on 23 April 2004.

Despite information of the availability of aural warnings being made available to members of the aviation industry, CASA understands that the "take up rate" for the installation of these devices is very low.

ATSB Comment:

The ATSB has classified this response as MONITOR, which means that the ATSB is keeping a watching brief on further safety issues related to cabin pressurization.

Last update 05 April 2012