Never Again Online: Denali's rough ride

March 1, 2006

On Monday March 7, 2005, as a pilot for Fly Denali Inc., I departed Talkeetna, Alaska, for a scenic flight around Mount McKinley with three passengers in a de Havilland DH-2 Beaver aircraft, N3307S. This was a Part 135 air-taxi operation. I filed a flight plan with the flight service station (FSS) at 10:38 a.m., Alaska Standard Time, for 1.5 hours of flight time — with three hours of fuel and four souls on board — to circumnavigate the mountain.

I received a weather briefing and current airport advisory from the FSS. There were no pilot reports for the back side of the mountain. There was an overcast layer at about 9,000 feet, and I proceeded to fly in to the Great Gorge of the Ruth Glacier and turned around in the amphitheater.

I asked and received the current Lake Minchumina and Healy weather. The north side was clear of clouds, and I headed north — up past the Eldridge Glacier. As I climbed to 11,000 feet, it was clear of clouds and the complete north side was open. It was totally clear and just awesome.

As I reached the area of the Muldrow Glacier around the Anderson Pass area, I leveled out and reduced rpm from 2,000 to 1,850. The airspeed was about 115 mph indicated and manifold pressure was 25 inches at that altitude. There was a small amount of turbulence crossing the range, and I told my passengers that the mountain was talking to us and instructed them to tighten their seat belts: It can quickly become very turbulent. Mount McKinley was completely visible and very nice.

I started to feel a small wobble in the yoke and looked out at the left wing. It was vibrating at about five oscillations per second, with 1 to 2 inches of deflection. It was similar to a bad shopping-cart wheel. The rudder pedals started to wobble uncontrollably, and I could not hold them with all the strength I had in my legs.

Within the next fraction of a second, the aircraft became completely uncontrollable — to say that it was shaking violently would be an understatement!

With deflections of the wings 2.5 to 3 feet outboard of the main lift strut, I watched the wing tips flap up and down so fast they were a blur, with the tip of the wings bending out of sight about midaileron. The noise was deafening. It was an "oil canning" sound with loud popping noises. My camera was flung from my jacket pocket to the floor between my feet. My passengers were frozen in fear and were holding on to the sides of the cabin. The shaking was so violent that it was hard to focus on any instrumentation. We were being bounced up and down in a vertical motion so rapidly, that I felt the cheeks of my face being pulled back and forth strongly. I had to clench my teeth so as not to bite my tongue. I slammed everything on the throttle quadrant to full-aft position — throttle, propeller, and mixture — killing the engine completely.

I simultaneously applied back-pressure on the yoke to slow the aircraft down and banked about 15 degrees to try to put a slight load on the aircraft, so I would not have such a steep angle of attack when pulling up. I depressed the push-to-talk switch and called Mayday, three times, "The airplane is coming apart; the airplane is coming apart; N3307S." It took both my hands on the yoke, which was bucking back and forth about 14 inches (not any side motion). I could not arrest it or hold it still. The rudder pedals were pumping back and forth as fast as they possibly could. The propeller never came to a complete stop. At about 50 mph the oscillations stopped completely.

I did not stall the aircraft at any time. I leveled the wings and brought the engine back online at 20 inches of manifold pressure and 1,600 rpm. I put in about 15 degrees of flaps and maintained less than 90 mph for the descent back to the east. At 90 mph, I started to feel the yoke vibrate and wobble again, and I reduced speed to 75. I made slight adjustments to the mixture and manifold settings as I descended and had an uneventful landing back in Talkeetna — not changing the flight configuration all the way to touchdown.

Upon examination of the aircraft, it was discovered that the wings had structural damage just outboard of the lift strut and halfway point to the empennage. The left wing had lost one fairing and popped all the screws out of the leading-edge fairings at the wing root. In conclusion, the insurance company deemed the aircraft a total loss. Both wings were damaged beyond repair. The right-wing aft attach point was completely severed and held in place by lift of the wing.

The FAA engineering team concluded that one aileron was 17 ounces out of balance and there was possibly a 0.003 discrepancy in a wing bushing that, when amplified to the length of the wing, was a contributing factor. General maintenance would not have detected this — everything was current and legal.

An FAA inspector who did the investigation stated — with no supporting data — that I had exceeded the aircraft's V NE speed and caused this to happen, a statement that caused me considerable hardship. I have since been totally exonerated from any wrongdoing, and I actually have been credited with a save of the three lives and my own, with my correct and swift response to the emergency.

Flutter is a very dangerous event, and any indication should be dealt with seriously. I'm glad that I was able to act quickly and nurse the aircraft back to a safe landing. I've also learned several important lessons.

Be alert to how your aircraft handles at all times. It is crucial to immediately recognize anomalies that could be signs of an impending structural problem. There was nothing during maintenance or preflight of the aircraft that indicated a pending problem with flutter, however I would advise anyone with this type of aircraft to have the controls balanced. Also, should there be any question of safe flight, act quickly and don't change the configuration until you have landed.


If you would like more information and the FAA engineering report, contact the author through his Web site.


Donald R. Lee, AOPA 1371674, is an Alaskan bush pilot who is the founder and operator of Alaska Floats and Skis. He is also a CFII who owns a Piper PA-20-160 that sports wheels, skis, and floats. And, with more than 12,800 hours accumulated during 30 years of flying, he has more than 3,600 glacier landings and 3,500 hours on floats, and is eager to share these experiences and pass on safety tips to his students and fellow pilots.


You can find additional information about coping with turbulence and mechanical malfunctions at the following links:


Look for the latest installment of "Never Again" in the April issue of AOPA Pilot. The story reveals how even a small trace of ice can render an aircraft uncontrollable.


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Posted Wednesday, March 08, 2006 5:01:12 PM