A92C0048 -Norontair De Havilland DHC-6-300 Twin Otter-Red Lake, Ontario 19 March 1992

Safety Action Taken

Air Carrier's Actions

Following this occurrence, the air carrier conducted an internal review of its policies, procedures, and training practices to assure their correctness. Also, the company pilots were directed not to use STOL procedures on the Twin Otter aircraft.

Transport Canada's Actions

Based on the information gathered during the initial stages of the investigation, Transport Canada advised its regional offices that all Twin Otter operators should be made aware that STOL operations outlined in the Supplemental Operating Procedures are not authorized in commercial operations.

Safety Action Required

Knowledge of Aircraft Flight Manuals

A single binder was used by the air carrier to hold the approved Aircraft Flight Manual (PSM 1-63-1A) and the non-approved Supplementary Operating Data Manual (PSM 1-63-1). This is an accepted practice in the aviation industry, and the combination binder is commonly referred to as the "Aircraft Flight Manual." Generally, a tab divider separates the two portions of the binder; in practice, little distinction is made between the "approved" and "non-approved" contents. It is not uncommon to find flight manual binders with the two categories of information inter-mixed, most likely as a result of years of local reproduction of the original manuals.

The mandatory or "approved" portion of the flight manual binder is generally comprised of the aircraft certification information relating to airspeeds, power plant and weight limitations, normal and emergency operating procedures, aircraft performance data, and loading parameters. The flight manual binder may contain supplemental operating procedures. Since this additional material has not necessarily been reviewed by the certification authority, it is referred to as "non-approved" information. Typically, this section includes acceleration-stop distances, specific range and cruise power charts, some climb and descent performance, system descriptions, maintenance procedures, and other handling characteristics. It often holds manufacturer-issued safety bulletins or communiqués covering topics from product enhancements to revised recommended operating procedures. However, at times, this "non-approved" information from the manufacturer goes beyond the "nice-to-know" category, by providing information essential for flight safety.

"Non-approved" material can generally be divided into two types: information which was not required for the certification of the aircraft, and information that did not meet the safety/performance level required during certification of the aircraft. The STOL procedures for the Twin Otter aircraft belong to the latter category.

The operator involved in this occurrence had separated the two categories of information in the flight manual binder. The company SOPs included duties for a 20-degree STOL take-off based on the "non-approved" procedure. Apparently, the company pilots considered this procedure as a "normal" training sequence, most likely unaware that the STOL procedure did not meet the safety criteria for commercial flight operations.

While there are guidelines on the use of "approved" information, there is no direction as to how the "non-approved" portion of the flight manual binder may be used by pilots and operators. Explanation of the differences between "approved" and "non-approved" information in flight manual binders is not required in initial or recurrent pilot training, nor is it found in commonly used flight publications. Hence, many in the aviation community are unaware of the differences. This lack of understanding regarding the proper use of the "non-approved" information contained in aircraft flight manual binders could result in pilots employing unsafe procedures. Therefore, the Board recommends that:

The Department of Transport promulgate guidelines regarding the use of "non-approved" material contained in aircraft flight manuals, including related operational information issued by aircraft manufacturers; and (A94-13)

The Department of Transport incorporate the subject of "approved" and "non-approved" aircraft flight information in the syllabus for initial pilot training. (A94-14)

Transport Canada's Response - A94-13:

Transport Canada Aviation agrees with this recommendation. An Aeronautical Information Circular (AIC) will be issued with the earliest available amendment cycle of the Aeronautical Information Publication (AIP) Canada. This AIC will go to all pilots and will explain the differences in the basis and use of approved and unapproved information.

Transport Canada's Response - A94-14:

Transport Canada Aviation agrees with this recommendation. The aforementioned AIC will be incorporated in the training for Private, Commercial and Airline Transport Pilot License candidates. In addition, questions concerning this material will appear in the applicable examinations.

DHC-6 (Twin Otter) STOL Performance

The Twin Otter aircraft has an international reputation as a STOL aircraft, and it is often employed in operations demanding short field operations. However, to indicate that the maximum performance STOL configuration of the Twin Otter does not meet certification standards, the manufacturer has provided a "NOTE" in the "Normal Operating Procedures" section of the Supplementary Operating Data Manual.

International aircraft publication standards of the General Aviation Manufacturers' Association and the Air Transport Association promote the use of a system of NOTES, CAUTIONS, and WARNINGS in flight manuals. A "NOTE" is meant to expand further on a topic; a "CAUTION" deals with matters that, if not strictly observed, could result in damage or destruction of equipment; and a "WARNING" directs attention to potentially critical information that, if disregarded, could lead to personal injury or loss of life. Although air crew may not know the exact definitions of NOTES, CAUTIONS, and WARNINGS, they are generally familiar with their relative importance through frequent exposure to them; that is, NOTES receive less attention than CAUTIONS and WARNINGS.

The use of the word "Normal" in the Twin Otter Supplementary Operating Data Manual to describe STOL procedures that do not provide the level of safety required by regulations for Normal Category Operations is misleading. Further, the message contained in the associated NOTE is more in line with what air crew would expect to be labeled as a WARNING.

As evidenced by this occurrence, the limitations of operating the Twin Otter in the maximum performance STOL configuration were not fully recognized by at least one Canadian operator. Consequently, Transport Canada issued a memorandum to its Regional Directors General on "the widespread use of unauthorized STOL procedures" for the DHC-6, alerting TC officials "to the hazards inherent in these procedures." The memorandum indicated "that operators should be made aware that the use of these procedures are not permitted" and "inspectors should ensure that these unapproved STOL operations are not being utilized by operators." Except for the internal Transport Canada memorandum, the Board is not aware of any formal communication or direction being given to Canadian operators regarding "non-approved" STOL procedures for the DHC-6. In March 1994, four randomly chosen DHC-6 operators in different regions across Canada indicated that Transport Canada had not contacted them regarding STOL procedures, and that STOL take-offs were being employed. Furthermore, given that the DHC-6 is widely used throughout the world, there does not appear to have been any related correspondence to international agencies responsible for foreign DHC-6 operators.

Since the DHC-6 Twin Otter is marketed as a STOL aircraft, since the information provided by the manufacturer regarding limitations on the "non-approved" procedures for maximum performance STOL take-offs is misleading, and since Transport Canada has advised its Regional Directors General that "the use of these procedures are not permitted," there is room for considerable misunderstanding in the global aviation community regarding the safe operation of this aircraft in the STOL configuration. Therefore, the Board recommends that:

The Department of Transport define, through a program of flight testing, the aircraft configuration and operating conditions under which "maximum performance" STOL take-offs are authorized; and A94-15

The Department of Transport take the necessary measures to advise all DHC-6 operators on a global basis of the operating limitations for these "authorized maximum performance" STOL procedures; and A94-16

Transport Canada's Response - A94-15:

Transport Canada Aviation is preparing an Air Carrier Advisory Circular (ACAC) which will further define the operating conditions for use of Maximum Performance STOL procedures. Only operators who have been authorized by an Operations Specification will be permitted to conduct STOL take-offs. Flight testing is not required to verify the aircraft configuration or operating conditions as testing was done by the manufacturer.

Transport Canada's Response - A94-16:

The DHC-6 Twin Otter certification basis, US Civil Aviation Regulations (CAR) 3, does not address Short Take-Off and Landing. As there is no domestic or international standard against which to qualify them, Maximum Performance - STOL procedures are not certified by the Department. They are published by the manufacturer in the unapproved section of the Aircraft Flight Manual (AFM). The approved AFM is clearly identified and separated from the unapproved or manufacturer provided Supplementary Operating Data. The manufacturer provided a clear note at the beginning of the section stating that since STOL capabilities “do not provide the level of safety required that since STOL capabilities “do not provide the level of safety required by CAR 3, Normal Category Operations, they must be used only when specifically authorized by the local regulating authority”.

Because DHC-6 STOL operations do not confirm to any internationally recognized standards, it is the prerogative of the aviation regulatory authority in each foreign state where the aircraft is operated to decide whether, and under what conditions, STOL operation will be permitted. It would not be appropriate for the Department to interfere in the jurisdiction of other states by communicating with foreign operators concerning these matters. However, the Department will send a letter to aviation authorities in foreign countries where the DHC-6 is operated to advise them of the circumstances of the accident and the measures that are being taken in Canada to improve regulation of DHC-6 STOL operations.

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