The clean wing—not just a concern in winter

The vast majority of GA pilots will no doubt have a firm understanding of the clean wing policy when it comes to winter operations.

The question we have to ask ourselves though, is do we realise that the same aerodynamic risk exists all year round?

Ice and snow contamination, depending on thickness and texture, will affect the efficiency of an aerofoil in different ways. The bottom line being that it affects it, so it is therefore important that before flight we remove anything that will impact aerodynamic efficiency.

Anything we or the environment do to our aerofoil will have the same effect as snow and ice, by disturbing what was otherwise a beautiful flow of air, from leading to trailing edge.

Some hazards are visible and easily identifiable on the walk around. A large number of bugs built up on the leading edge, for example. What is the difference between this and impact ice in terms of changing the shape of the leading edge?

Wing
There’s no ice, but the wing definitely isn’t clean.

Hangar rash and bird impact damage is always a subjective topic, as often a manufacturer will provide maintenance manual tolerances and allow an aircraft to remain in service. Whilst on a general performance level it may fall within acceptable limits, by definition the shape of the aerofoil has changed in that section.

I was once tasked to conduct a flight test on a business jet at a completion centre due to the fact that the aircraft had developed a very pronounced wing drop at the stall.

Sure enough, I found that the left wing dropped extremely violently and was outside of production flight test limits, whereas prior to the maintenance visit it had behaved impeccably. Obviously something had changed and it had to be aerodynamics.

Looking through the maintenance records, the clue was buried in the reams of paperwork. A wing anti-ice squawk had required the left leading edge to be removed and re installed as part of the troubleshooting and rectification.

When I ran my hand along the joint between the leading edge and wing skin, I immediately felt a “ridge” from excessive sealant. That ridge ran span-wise from wing root to wing tip, top and bottom. It was only measurable in millimetres protruding into the airflow, but the effect was devastating. Removed and resealed flush, the aircraft regained its composure with docile stalls.

We must never forget also that elevators and horizontal stabilizers are just as capable of stalling. So whilst it looks cool to run around in a Jeep caked in mud, allowing the tail feathers to look as if they have been in a fight with a Hershey bar can end in tears.

The most bizarre contamination of an aerofoil I ever experienced was after a day of operating in and out of ranch strips. The following morning when I went to get the 182 out, the stench from the hangar was overwhelming.

My nose eventually led me to what looked like and smelt like skunk meat, all over and stuck to the top wing!

No evidence of a prop strike or blood, no evidence of a bird of prey having used the wing as a picnic table.

To this day, whenever I do a walk around, I still look for aerofoil contamination, regardless of the season. Sometimes you just need a good sense of smell!

9 Comments

  • I recall an aerodynamacist from Canadair (it may have already been Bombardier by then, I can’t recall) tellling me that they had had great difficulty with the certification stall testing of the CRJ-100, with a hard wing, because they were doing it during black fly season in Canada. As I recall, he said no matter what they tried, by the time they had completed the takeoff the wing was contaminated, and the stall tests resulted in data that was not, well, quite what they wanted to publish…

  • Many thanks Dave and Steve for your comments. The tragedy at Birmingham, England where the sun melted the thin hoar frost on one wing and not the other on a Challenger, shows just how critical a clean wing is as Alpha is increased during critical phases of flight such as rotation. ( Not De-iced). More information on Flight Test and certification is covered in a book I published last year you may find of interest. Steve.
    https://20westbook.com/

  • I’m sending your article to CAP Safety for crew members to read. It has been something I have been preaching for years. Clean wings are needed year round not when the weather goes below freezing.

    One hangar we keep a plane in is notorious for pigeons and there is always crap on the wings and horizontal stabilizers to the point if I had to fly it, it gets washed first. Nobody seems to mind the crap, but will not fly it if frost is on it. Doesn’t make sense to me. The plane needs to be clean either way.

  • In the late 1970’s one of my students bought a Cherokee 140 that needed paint. We flew it a few times before I suggested he strip the paint off just the top of the wings since it had gotten to the point of curling and flaking. He gained about 6 knots and a bunch of climb performance just from taking off the flaky paint.

  • That is a really good point Dan and another area often over looked. Anything that protrudes into the airflow will disturb it, including curled or flaky paint.

  • I can recall many times I flew my Cherokee 160 to my A&P’s hangar for annuals or maintenance. He would get the cows off the runway before I headed over. You can probably imagine the looks (and smell) of my wings and stabilator after landing. The brown (and green if they were eating rye) splatters certainly made it “not and option” to clean up quickly, so never an issue! 🙂

  • Probably the cleanest Cherokee wings in the county! That is the last thing you would ever want to bake on in the summer sun.

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