AIR CARRIER ADVISORY CIRCULAR
Notice To Operators Of DeHavilland DHC-6 Twin Otter Aircraft Maximum Performance STOL Takeoff - (Limited Distribution)
Several operators of the DHC-6 Twin Otter aircraft have requested the use in commercial operation of the "Maximum Performance STOL" (MPS) supplementary operating data detailed in the Aircraft Flight Manual. The purpose of this Air Carrier Advisory Circular(ACAC) is to set out the guidelines for DHC-6 MPS takeoffs.
Transportation Safety Board of Canada (TSB) investigation into a DHC-6 accident in March of 1992 revealed that several operators were conducting MPS takeoffs in this aircraft despite the unapproved status of the procedure. The first page outlining the procedure bears an annotation requiring specific authorization by the regulating authority for MPS operations. Because no such approvals had been granted or requested at the time of the TSB recommendation, Transport Canada Aviation (TCA) Air Carrier Inspectors were directed to advise all affected air carriers that MPS takeoffs should cease until approval was granted.
Subsequent to that action, TCA began researching DHC-6 MPS operations together with industry pilots and Flight Safety International's DHC-6 simulator staff in order to determine a safe mode of operation. A secondary goal was to evaluate the effectiveness of the simulator in MPS training.
Exercises included all-engine MPS takeoffs, and the following engine failures:
- critical engine failure at liftoff, autofeather operating;
- critical engine failure at liftoff, autofeather off;
- critical engine failure at Vmca, autofeather operating;
- critical engine failure at Vmca, autofeather off;
- critical engine failure at liftoff during single-pilot MPS operations; and
- failure of the non-critical engine at liftoff, autofeather operating.
The research yielded the following results:
during MPS operations, the aircraft becomes airborne well below Vmca. With both engines operating, acceleration to Vmca occurs in 1-3 seconds. This creates a window during which an engine failure could result in a loss of directional control, particularly if the inoperative engine does not feather immediately;
initial climb following a MPS liftoff is very shallow, requiring special obstacle clearance considerations;
even when properly flown, a single-engine climb following a MPS lift-off resulted in a lateral deviation of between 15 and 30 degrees from centreline. At 0.5 NM, this resulted in lateral displacements of over 500 feet from the centreline. Altitude gain did not exceed 160 feet at 1 NM from the point of liftoff with takeoff weights from 10,800 pounds to 12,500 pounds;
use of autofeather greatly enhanced initial climb performance following an engine failure. In cases where autofeather was not used, the aircraft generally did not perform adequately to ensure a safe recovery;
- the simulator used provided an adequate facility for conducting MPS training and testing.
Operations employing the MPS takeoff procedure will require specific authorization in the form of an Operations Specification from TCA. Single pilot MPS operations will be authorized separately from MPS operations using two-pilot crews.
MPS takeoffs may be conducted by an authorized operator provided the following conditions are met:
the Chief Pilot or senior pilot responsible for DHC-6 training will require a minimum of two hours MPS familiarization in a simulator equipped to faithfully recreate MPS normal and abnormal operations;
line pilots who are authorized to conduct MPS takeoffs will require annual training consisting of the following:
ground training covering MPS normal and abnormal procedures for each series of aircraft operated;
a review of Limitations during normal and MPS operations; and
- a minimum of three MPS takeoffs monitored by an individual qualified in accordance with paragraph (a) immediately above.
Note: Engine failures associated with MPS conditions shall not be simulated below 1000 feet AGL.
- ground training covering MPS normal and abnormal procedures for each series of aircraft operated;
proof of annual MPS training will be indicated on the pilot's company training files and certified by an individual qualified as indicated in paragraph (a) immediately above;
company MPS procedures will be set out in the Operations Manual;
for each MPS departure:
the aircraft shall have a fully serviceable autofeather system installed and operating;
the pilot-in-command shall verify that adequate visual cues exist to conduct a MPS takeoff. MPS takeoffs shall not be performed at night nor in whiteout/greyout conditions;
- the pilot-in-command shall ensure that the ceiling and visibility are adequate to permit visual obstacle avoidance in the event of an engine failure during a MPS takeoff.
- the aircraft shall have a fully serviceable autofeather system installed and operating;
This ACAC has identified the conditions necessary for the use of the "Maximum Performance ADAC" takeoff procedure which appears as Supplementary Operating Data for the DHC-6 Twin Otter aircraft. Operators wishing to employ this procedure should contact the nearest TCA regional office.
Commercial & Business Aviation Advisory Circulars (CBAAC) are intended to provide information and guidance regarding operational matters. A CBAAC may describe an acceptable, but not the only, means of demonstrating compliance with existing regulations. CBAACs in and of themselves do not change, create any additional, authorize changes in, or permit deviations from regulatory requirements.
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