Research on the Stinson 108-3 Flies Through Trees at CML2 Video

Ok, this is a Stinson 108-3, which like;y has a 165hp Franklin engine and a max gross weight of 2400 lbs with a factory empty weight of 1320lbs. Fuel capacity was 50 gallons. Being these aircraft are often heavily modified, its hard to tell how legit those numbers are.

The airport is CML2, which has a field elevation of 130ft MSL and its located just north of Victoria BC. The lone runway is 1800x75ft. Maximum recorded temperatures in the area are under 100F. On the day of this flight, the pilot said it was hot… so giving the benefit of a doubt, lets go with 100F and moderately humid at say 70%. Such would put density altitude at around 3300 ft which will increase the length of the take off roll.

The airplane has 3 souls on board, all of which are pilots, so lets assume each weighs 170 lbs. We don’t know the fuel loading, but since fuel was not available on the airport, the tanks cannot be full. Lets figure around 35 gallons, which works out to be around 210 lbs. Alas, even though its under max gross weight at ~2100 lbs (includes some additions like shoulder belts, avionics, dust/dirt, fabric, dope, etc over the years), it will not perform anywhere near as well as the idealized factory values.

On a positive note, the runway slops downhill, so even though it is an unimproved surface, take off roll will decrease, assuming winds are aligned with the runway.

As this is an older and potentially highly modified aircraft, online performance charts are lacking. Stinson states the take off roll under unknown conditions to be 980ft. Some pilots suggest fully loaded under unknown conditions it is 1500ft. Being the aircraft did achieve flight, despite improper pilot technique, a calculated take off roll under these conditions would likely fall somewhere between the two figures, lets say 1250 ft.

Bottom line, taking off on this 1800 ft runway under the above assumptions provides for a ~45% margin. (1800ft runway-1250ft takeoff roll)/1250ft takeoff roll.  As a result, its pretty unlikely that the other pilots on board questioned the pilot in command as to whether the flight was safe or not.

Alas, this presents an interesting question, one of which many of my students have asked over the years… what sort of margin should I add to my performance figures? In this case, 45% almost resulted in an accident. Being 30-50% is a pretty common answer I’ve gotten back from fight reviews over the years, such is a common buffer for a lot of private pilots, but is it really the right one?

I remember as a young CFI asking an old bush pilot this exact question, having had my own near miss at the end of a runway with too small a margin. His answer, and the one both he and I go with for personal flying… 100%!

More on this in an upcoming post.


Sources

Density Altitude Calculator

Humidity Dewpoint Calculator

Stinson 108-3 Specifications

CML2 Airfield Data from Skyvector

Commentary from the pilot in command in the above video

I really appreciate that the pilot in command provided some commentary on this. Its far too easy to sit an an armchair and quarterback this type of thing as a “this will never happen to me, I’m not stupid” that is, until it does. Bottom line, if NASA with all their safety checks and technical acumen can blow a units conversion and crash a satellite, who are we to think we are immune to calculation and/or judgment errors when it comes to aviating.

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