We’ll never know. In a report of the accident filed with the Aviation Safety Reporting System, the pilot parsed a laundry list of possible causes—patchy ice on the runway, taking off on the airport’s more-convenient-but-shorter runway, no recent experience in the aircraft—but the narrative kept coming back to frost.
“Having always had wing covers on [aircraft] I’d flown previously, I was a little uncomfortable with the frost on the wings. I brushed it very well; but it was still a concern in the back of mind,” the pilot wrote in one of several musings.
Those never-to-be-resolved concerns capture the essence of the frost hazard: Just a little bit can hinder or preclude flight in two ways. “Frost disrupts the flow of air over the wing and can drastically reduce the production of lift. It also increases drag, which when combined with lowered lift production, can adversely affect the ability to take off,” notes the Pilot’s Handbook of Aeronautical Knowledge (page 12-15). “An aircraft must be thoroughly cleaned and free of frost prior to beginning a flight.”
It’s apparent that the pilot was aware of those general notions about frost but remained uncertain about how much was too much.
The AOPA Air Safety Institute’s “Aircraft Icing” Safety Advisor takes on that question citing research. “Wind tunnel and flight tests have shown that frost, snow, and ice accumulations (on the leading edge or upper surface of the wing) no thicker or rougher than a piece of coarse sandpaper can reduce lift by 30 percent and increase drag up to 40 percent. Larger accretions can reduce lift even more and can increase drag by 80 percent or more.”
If a sandpaper-thick accumulation can have a devastating impact on airfoil efficiency, it stands to reason that a lesser accretion must also be avoided by completely removing the frost.
As for using a short runway with poor braking conditions when in doubt about whether the aircraft will fly—that’s a separate discussion.
Meanwhile, with fall surface-air temperatures cooling down, taking dew points along with them, consider yourself in the frost zone once those values dip below freezing—perhaps coming soon to an airport near you.