Icing: test flight performance
Original post: 4/25/2002
By: Bob Kromer
One of the finest postings I've ever seen on this chat line was the one on the icing encounter in the M20C. It's information like that that makes this line of communication what it should be. Factual, detail oriented information that we can all use backed by actual "hands on the controls" experience. Great job!
The photos reminded me of an engineering flight test evaluation I flew at the factory in January, 1985. I was dispatched in the factory's heavily instrumented M20K 231 prototype from Kerrville to northern Alberta, Canada to investigate engine induction system icing. We operated out of Ponoca, Alberta (just south of Edmonton) and flew daily flights out west over the Rocky mountains in the direction of British Columbia and back.
Needless to say, I found lots of induction system icing and these tests resulted in a major retrofit to the 231's alternate air induction system that everyone should have incorporated ( it was provided free by the factory). But as you can imagine, we also learned a lot about structural icing on the Mooney airframe during thest tests. Lots of important information came from those tests, but I want to pass along what I think was the most important for your consideration.
Most of the airframe ice I accumulated at altitude was brought to the approach and landing. It was just too cold at lower altitudes to melt anything. And what we learned that I think was so very important was this - WITH AIRFRAME ICE ON A MOONEY DURING APPROACH AND LANDING, CONSIDER LIMITING FLAP DEFLECTIONS FOR THE LANDING.
Here's the reason. We all know the amount of nose up trim required in our airplanes is pretty substantial to retrim when the flaps are extended - especially the M20K, M20M, M20S and M20R airplanes. Retrimming our airplanes causes the entire horizontal stabilizer to move and significantly changes the angle of attack of the horizontal tail's leading edge (nose up trim is leading edge down). With a substantial amount of ice on the horizontal tail (1/2 inch or greater), we found that the airflow could seperate over the horizontal tail and the horizontal tail could begin to stall when retrimming the airplane nose up for the landing with anything greater than 15 degrees of flap deflection.
The buffeting started around 85 KIAS with the flaps at the takeoff setting (15 degrees) and got worse as we got slower. Buffeting was felt through the control column and elevator effectiveness was greatly diminished. With ice on the M20K prototype while landing, I learned to make my approach and landings at 90 KIAS with the flaps up. This completely avoided the retrimming/tail buffeting/stall situation. We felt so strongly about this (limiting flap deflection for landing with ice on the airframe) that we immediately added this recommendation to the Pilot Operating Handbook for the 1985 model M20K when we got back.
The aerodynamics of the airplanes we all fly are very similar. I suspect that what we experienced on the M20K prototype in Canada you will experience in your airplane. So think about limiting flaps to minimize retrimming the tail when landing with airframe ice on your Mooney. Keep the speed up on the approach (85-90 KIAS) and carry that extra speed to the landing flare (just make sure the runway is long enough to handle the extra speed and landing distance). Stalling the horizontal tail is something not to be reckoned with, especially close to the ground.
Again, these are just some recommendations based on flight testing I was fortunate enough to participate in while at the factory. Please, if you have a better idea or method that has worked for you, wonderful. Always go with what has worked for you. Remember, no one person on this list knows everyting about how to fly a Mooney. There is always something to learn. But together, sharing information and experiences we have all encountered in the Mooney cockpit, we can learn so much about the wonderful airplanes we love and fly.
Best Regards,
Bob Kromer
Revision: 10/28/2010