I cannot comment on the LET, never having flown one, but I can comment with some authority on the Twin Otter, having written the book on the aircraft (the FlightSafety Training Manual & Checklist), spent 5,000+ hours training Twin Otter pilots, and having considerable experience flying them in a variety of places, Hassi included.
First, a few statistical corrections to earlier posts: There were 844 Twin Otters built, the production run was as follows:
- 6 pre-production prototypes, model designation DHC6 Series 1 (2 of which are still flying, one in service with NASA).
- 109 Series 100 aircraft, which was the first production run. (The total of 115 given earlier includes the 6 Series 1 aircraft). Most of the few remaining Series 100 aircraft still flying are used for recreational parachute dropping.
- 115 Series 200 aircraft.
The balance (SN 231 to 844) were Series 300 or derivatives. Slightly less than 600 remain in service today. Series 300 have a MTOW of 12,500 lbs, the earlier series are limited to 11,596 lbs MTOW.
Concerning the derivatives built during the production run – series 110, 210, and 310 conformed with UK certification regulations (minor electrical differences, mostly), and Series 320 conformed with Australian certification regulations (minor avionics differences). 310 and 320 Series aircraft can be operated as if they were 300 Series, in accordance with the 300 series AFM, if so desired, provided the regulatory authority does not require conformance with UK or Australian regulations.
The two ‘Killer’ Twin Otters made – the 300M series – were both sold to Senegal for coastal fisheries patrol. As with the 310 and 320, they were part of the regular Series 300 production, modified as needed. One has since been returned to civilian use as a Series 300 aircraft. There were also a small number of Series 300 aircraft that were fitted with enhanced avionics, wing spoilers, dual zone engine fire detection, anti-skid, and an electrical system that conformed with FAR 25 for a STOL air carrier demonstration project in Canada. These were eventually sold to Transport Canada for utility use, and all were converted back to the 300 spec. Transport Canada has since sold these, they are out in the civilian world now.
The Canadian Military purchased some DHC-6 300 aircraft for utility use, and assigned them their internal reference number CC-138. They are nothing more than normal series 300 aircraft with slight modifications as requested by the customer – a roll-up door, bubble windows for SAR use, and so forth. The United States Air Force purchased 2 series 300 for parachute dropping – they still operate them, and designate them as the UV 18.
All the aircraft described in the above two paragraphs conform to the civilian type certificate for the DHC6 300, the differences requested by the customers having been incorporated by way of engineering orders only.
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In 1998, I made a study of all DHC-6 accidents and incidents from 1965 to 1997 – about 600 events in total. It is interesting to observe the ranking of accidents by type and accidents by causal factor (classified by ICAO standards).
The 5 leading accident causes by type for that time period were:
1) Loss of Control - Ground or Water (69)
- during takeoff or landing phase, about an equal split
- unsuitable terrain the most common factor
2) Collision with Rising Terrain (59)
- CFIT, almost always multiple fatalities
- pressing the weather, VFR flight to IMC destination
3) Collision with Objects (54)
- during taxi, takeoff or landing
- includes 30 accidents related to hydraulic CB pulled
4) Heavy Landing (48)
- often includes stalls, attempts at STOL landings
5) Collision with Ground or Water (31)
- hit the ground or water during approach.
The top 5 causal factors – for the same group of accidents – were:
1) Continued VFR flight into deteriorating weather (52)
- Pre 1990 - CFIT enroute to destination
- Post 1990 - CFIT near destination after GPS navigation enroute
2) Unsuitable terrain (34)
- inadequate runway surface
3) Misjudgment of distance (34)
- undershoots and overshoots
- hitting trees on approach or takeoff
4) Improper landing technique (23)
- stalls, or carrying power into the flare to try and get a “greaser”
- props not forward prior to landing
5) Failure to compensate for wind (22)
- landing or takeoffs in thunderstorms, crosswinds too great
- groundloops (!)
- blown off runway into trees, ditches, etc
- short runways and strong crosswinds at the same time.
I think it is fairly clear that the bulk of DHC-6 accidents arise from attempting to operate the aircraft in an environment that is simply not suitable for an aircraft – any aircraft. I don’t think there have been many Boeing, Airbus, or Gulfstream aircraft damaged as a result of ‘Takeoff or landing attempted on unsuitable terrain’ or ‘blown off runway into ditch’.
Accident rates for the DHC-6 have declined sharply since the mid 1990s. One of the reasons for this is the introduction of the DHC-6 simulator in Toronto in the early 1990s. A second reason is the general tightening up, worldwide, of supervision of remote and utility operations by either clients or the regulatory authorities. The bulk of accidents that have taken place since the late 1990s can be attributed to pilots not following the procedures published in the AFM – pilots pulling circuit breakers, or not using full calculated power for takeoff, or landing with the props in the minimum governing position. There have only been 2 DHC-6 accidents over the past 40 years attributed to mechanical failure of otherwise properly maintained aircraft.
‘let410fly’ makes reference to performance calculations and data relating to single engine flight. There are two different sets of certification data available for use with the DHC-6 300 series. One is the original CAR 3 data, the other is the SFAR 23 certification data, which was published after the aircraft had been in service for some time. The SFAR 23 data does provide all that is needed to make a well informed decision about runway length required for accelerate-stop and single engine climb performance following V1. That data can be found in Supplement 11 of the AFM. The operator has a choice of using either the CAR 3 data, or the SFAR 23 data, to govern their operations. Most operators now choose to use the more conservative SFAR 23 data.
With respect to the Twin Otter being a 'handful' in heavy crosswinds, keep in mind that the stalling speed of the aircraft in the landing configuration can be as low as 50 to 55 knots. If you have a 25 knot crosswind, that's a 50% crosswind component. I am sure that a Boeing with a stalling speed of 120 knots in the landing configuration would also be quite a handful in a 60 knot crosswind - the same percentage. The low stalling speed gives the Twin Otter unique advantages, but as pilots, we have to remember that the aircraft reacts to the crosswind component, not the crosswind in knots.
As I mentioned at the beginning of this post, I have no knowledge about the LET, and for that reason, I cannot comment on it or draw any comparisons.
Michael