|Wood Wing Mooneys - Are They Safe?
AN INTERVIEW WITH BILL WHEAT
By Bob Kromer, MAPA President
Here at MAPA's headquarters, we get at least one call a week from someone who has spotted a wood-wing Mooney for sale at a bargain basement price (usually less than $30,000) asking us is the airplane still safe to fly or not. We thought you might be interested in what Bill Wheat has to say on this subject. For those of you that don?t know Bill, he has spent the last forty-two years at the Mooney factory in a variety of technical and engineering positions. No one knows the pre-J model airplanes like Bill. I consider him the top authority on technical matters pertaining to pre-J model airplanes, including the wood wing airplanes. Here is what Bill had to say during an interview I conducted with him recently on the subject of wood wing Mooneys.
Bob - Bill thanks for taking the time to talk to me today on the subject of wood wing Mooneys and their continued airworthiness. First of all, from your records how many wood wing airplanes were manufactured by the Mooney factory?
Bill - We built wood wing airplanes in Kerrville from 1956 thru 1960. The 150 horsepower M20 was built in '56 and '57. My records show a total of 199 M20's produced. The 180 horsepower M20A was made in '58, '59, and '60. We built a total of 499 of these airplanes before metalizing in '61. So, that makes a total of about 698 wood wing/wood tail Mooneys manufactured.
Bob - How many are still flying today?
Bill - Our records show about 350 still in airworthy condition.
Bob - Exactly how much wood was used on these airplanes?
Bill - People forget that along with the wings, the empennage was also built of wood. All M20's and M20A's were built this way originally from the factory - wood wing, wood tail.
Bob - So, why is it that most of these airplanes you see today have a metal tail?
Bill - It's because in 1968 the factory released Service Bulletin M20-170 (later revised to 170A) and the FAA released AD 68-25-6 making M20-170 mandatory. In effect, M20-170 stated "Installation of a metal empennage is mandatory within one year". As a result, most of the wood tails have long since been replaced with metal ones.
Bob - Are there still some airplanes out there flying with wood tails?
Bill - Yes, but I would consider these airplanes unairworthy. Those original wood tails just weren't meant to be flying this long. If any of your members is considering the purchase of an M20 or M20A that still has a wood tail, tell them not to buy it because of the tail.
The way they used to build 'em. This photo (circa 1957) shows the buildup of the wood wings.
Bob - Bill, you've flown lots of hours in the M20 and M20A. Tell me what you thought of those airplanes in their day. Also, can you tell our members why the factory decided to build the airplanes out of wood in the first place?
Bill - I have to tell you, I think that the wood wing airplanes in their day were probably some of the best airplanes we ever built in Kerrville. I know that sounds old fashioned, but let me tell you why I think that way. First of all, for equal horsepower, the wood wing airplanes were faster than the metal ones. And not just a little bit - we measured a speed loss of anywhere from 5-8 mph when we converted the wood wing M20A to the metal wing M20B. That's a lot of performance loss. The reason the wood wings were faster is simple - aerodynamics. A well built Mooney wood wing is a thing of beauty. It has a glass-like finish with almost perfect contours. The airfoil is very true and the leading edge is perfectly shaped. This results in low drag and high speed. Thus the speed difference between wood and metal.
Another factor in favor of the wood wing is the quality of the ride in turbulence. The ride of a wood wing is better than a metal one. Those of us with lots of time in wood wing Mooneys noticed right from the start that the quality of the ride in the metal wing 1961 B model was harsher and stiffer than the wood wing 1960 M20A model. Finally, the stall characteristics of the wood wing were much better than the metal ones. Because the airfoil was truer with the wood wing, the old M20 and M20A had consistent and predictable stall characteristics. Not so with the metal ones. We are always fooling around with the location of the stall strips on the leading edges of the metal wings during production flight test to get acceptable stall characteristics. With the wood wings, stall characteristics were so good that we could permanently install the stall strips into the wing leading edges while the wings were originally built and leave them there. And every wood wing stalled beautifully.
Bob - So, why did the factory want to metalize in 1961 and do you agree it was a good idea?
Bill - Absolutely, it was a good idea and the only way for the Mooney to be accepted as a modern airplane as the 1960's began. There are really three reasons why the change was made from wood to metal. And all three are good ones. First and foremost was the marketing aspect. People just believe that a metal wing will last longer than a wood one. People think of rot when they think of wood. When they think of aluminum, they think it will last forever.
The second reason for the change to metal was that we lost our master woodworkers from the factory as the '60s began. Those men and women who knew how to work with wood learned their trade in the 1920's and 1930's. As they begin to retire in the late '50's and early '60's, we were left without anyone knowing how to make wood wings and tails. We had no choice but to redesign using metal.
The third reason was ease of repair. As mechanics in the field began to retire who knew how to repair wood, it became harder and harder to find someone who could repair your wood winged Mooney if you damaged it. So the switch to metal made it a lot easier for Mooney owners to get repairs done in the field.
Bob - Are there many of these mechanics with wood expertise left out there?
Bill - Not many, but I'll give you the list that I have of shops that specialize in maintaining the wood wing Mooneys. It is imperative that your members who own and fly these airplanes get in touch with one of these shops for routine maintenance and inspections. This the only way, I repeat, the only way, these airplanes can be kept airworthy - by undergoing regularly scheduled maintenance by an individual or shop that understands wood and knows how to inspect and repair it.
Bob - Bill, we are aware of several in-flight breakups of wood wing Mooneys in the past several years. Most were associated with some type of high "G" maneuver followed by a failure of the wing spar. From these accidents, are there certain areas on a wood wing that should be specifically inspected on a regular basis for structural integrity?
Bill - Yes, let me give you some basic guidelines on what areas should be inspected. First, you'e got to pull the rear gas tank under the rear seat and inspect the wing spars in that area (in the bottom of the airplane). Specifically, look at the wing-fuselage attach points on the spars in this area for rot. Any water leaking outboard of the fuselage around the wing/fuselage fairings will settle in this area and could cause wood rot. Another problem area is along the trailing edge of the wings in front of the flaps. There is a flap gap seal on the top where water can soak down into the wing in front of the seal and cause rot. Another thing that must be watched for carefully is that all drain holes in the wing bays are clear and allow water to pass. I can't tell you how many cases of wing rot would have been avoided if the drain holes were simply kept clear and clean. Also, as we discussed before, do not consider any airplane that still has a wood tail. If a wood tail has rot, it can't be repaired and is a structural failure waiting to happen.
If the airplane you are considering still has a wood tail - forget it.
Bob - So, let's summarize what we've said here concerning wood wing airplanes.
Bill - The bottom line is that these airplanes can be flown safely only if they are maintained by a specialist who knows how to inspect wood aircraft structures for rot and knows how to repair wings where rot is present. That said, I personally consider woo d wing Mooneys as classics - some of the best airplanes we ever built at the factory. They are faster than the metal airplanes, ride better in turbulence and have excellent aerodynamic characteristics.
Bob - Would your personally own and fly a wood wing Mooney today?
Bill - Would I? Yes, but it would have to have a metal tail. And it would have to be maintained and inspected often by a shop that specializes in wood aircraft structures and has specific knowledge of the early Mooney airplanes. Remember, these airplanes are now 40-45 years old with wings made out of wood. Without an individual or shop that knows wood aircraft structure looking after my airplane, I could be flying on a rotten wing. Sooner or later, a rotten wing will break. And that scares me to death for your members who aren't getting these airplanes inspected properly - and often.
on Wood Wing Mooneys
1. Wiebe Aircraft
P.O. Box 58
Halstead, Kansas 67056
Gus and Mark Wiebe
2. B&D Aircraft
Houston Southwest Airport
503 McKeever, #1502
Rosharon, Texas 77583
3. Rio Grande Aero Services
42 Elaine Drive
Mid Valley Airpark
Los Lunas, New Mexico 87031