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How Frost Contributed To A Cessna 172 Takeoff Accident

Boldmethod

It's officially fall, and that means frost is on the way. It's rare to see a takeoff accident where frost is the only contributing factor. But add one or two other factors into a takeoff, and frost can play a major role.

This Cessna 172 takeoff accident in Naknek, AK (5NK) is a prime example. Let's start with the NTSB's report.

Analysis

The private certificated pilot was departing uphill from a 1,950 feet long runway on a Title 14, CFR Part 91 local area personal flight. The pilot indicated that prior to departure, the upper wing surfaces of the airplane had a layer of frost, but he smoothed the surface with a broom. During the takeoff roll, the pilot said the airplane became airborne in ground effect to an altitude of about 2 feet, about 300 feet before the end of the runway. The airplane stalled and collided with alder bushes about 75 feet beyond the end of the runway. The airplane received damage to the wings, fuselage, and engine.

Probable Cause and Findings

The National Transportation Safety Board determines the probable cause(s) of this accident to be: The pilot's inadequate removal of frost from the wings, and his failure to attain adequate airspeed during takeoff initial climb, which resulted in a stall and collision with terrain. Factors contributing to the accident were the pilot's delayed aborted takeoff, and an inadvertent stall.

Three Factors At Play

The NTSB cites improper frost removal as the probable cause of the accident, but there were really three factors at play. In addition to the frost, the takeoff was uphill, on a relatively short 1,950' gravel runway.

Upslope takeoffs increase takeoff distance significantly. While the Cessna 172 POH doesn't provide guidance for upslope takeoffs, the Cirrus SR-22 POH says that for every 1 degree of upslope, takeoff distance increases 22% at sea level (5NK airport is 70' MSL).

On top of that, the runway is gravel, which increases takeoff distance compared to smooth, dry pavement. The 172 POH doesn't provide takeoff data for a gravel runway, but it does for dry grass: a 15% increase in takeoff distance. And while gravel doesn't create as much drag as grass, it's not as smooth as pavement, which again increased the 172's required takeoff distance.

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How Much Can Frost Affect Your Plane?

Finally, let's look at how the broomed frost could have affected takeoff performance. According to the NTSB:

  • Frost the size of a grain of salt, distributed as sparsely as one per square centimeter over a wing's surface, can destroy enough lift to prevent your plane from taking off.
  • Frost can reduce your wing's max lift by 30 percent or more.
  • It can also reduce your wing's critical angle-of-attack by several degrees.
  • Because frost disrupts airflow over your entire aircraft, it can increase drag by up to 40%.

Just under a year after the accident, the FAA updated their guidance on takeoff with polished frost in SAFO 06014. Here's an excerpt from the Safety Alert:

Discussion: Since 1960, operational experience and accident history have shown that contamination of any kind can adversely affect the aerodynamic properties of an airfoil, and that the safest course of action is to completely remove all contaminants from wing and flight control surfaces.

Therefore, the FAA cannot support the practice of merely polishing frost on a wing or control surface unless an aircraft manufacturer has developed explicit, approved procedures for doing so, and these procedures are strictly adhered to in operations and supported in training.

Recommended action: Pending rule changes, directors of operations, directors of training and pilots should ensure (1) that during operations in ground icing conditions no contaminants including frost are adhering to wings or to stabilizing or control surfaces immediately prior to takeoff; and (2) that "polishing frost" as a means to meet this objective is not practiced unless an aircraft manufacturer has developed explicit, approved procedures for doing so, and these procedures are strictly adhered to in operations and supported in training.

Something We Can All Learn From

Unfortunately, we don't have all the data from the takeoff. We don't know what the winds were, or how close the aircraft was to max gross weight, both of which which play a significant role in takeoff distance.

What we do know is the 172 was airborne in ground effect, but wasn't able to climb any further in the 1,950' of runway distance available. Had the plane taken off from a longer runway, it's entirely possible that it would have climbed out.

The combination of frost, an upslope takeoff, and a short gravel runway was more than the plane was capable of. Take one or two factors away, and the accident most likely would have never happened.

Colin Cutler

Colin is a Boldmethod co-founder, pilot and graphic artist. He's been a flight instructor at the University of North Dakota, an airline pilot on the CRJ-200, and has directed development of numerous commercial and military training systems. You can reach him at colin@boldmethod.com.

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