Monday, January 31, 2011

More on Landings - deck angle of the tricycle rigged aircraft during landing

A problem I commonly see in landings is that a pilot pulls on the yoke often reaching a proper landing deck angle soon after the round out. Ok so far, but the next segment of the landing is where things go haywire: the pilot does not maintain this deck angle before the wheels touch and this is a mistake.

What is deck angle? I use it here as slang for the angle of the longitudinal axis of the aircraft from the landing surface, which usually means a runway surface.

To illustrate, assume that the XYZ make and model needs to be landed with a deck angle of 7.5 degrees. Right after the round out, the pilot flares to 7.5 degrees and then freezes - no more pulling on the yoke. What happens to the deck angle? Why? Answers: the deck angle decreases because the effectiveness of the elevator in maintaining pitch decays as the airspeed decays so the nose keeps dropping until a flat landing occurs - simple aerodynamics. In other words, to maintain proper deck angle, the pilot must keep pulling to increase elevator deflection to obtain a constant angle. Instead of a constant landing deck angle, this angle constantly decreases until the wheels touch. This means a poor transfer of weight from wing lift to wheels and it also means a lot more ground speed when the wheels touch. The former is hard on the airframe and the later   unncessarily increases tire wear.

Knowing deck angle in degrees is asking a lot out of most GA pilots and who wants to watch an AI during a landing - not me! There are indices that often work well in the cockpit. For example, for many trainers (not all!), proper deck angle can be acquired by flaring until the front end of the engine cowling is high, but no so high that the runway center line is covered by the cowling. For these aircraft, this means that the nose is really high, but the pilot can always see some center line markings off the very end of the cowling - not a lot, just some. It's my experience this is true for the following aircraft and perhaps more: The C152, PA28-161 & 181, C172(N,S), C182 (all models I have flown). 

Many pilots who do not maintain a proper deck angle are often uncomfortable with a proper landing because they feel the pitch is way to high and looking out at a lot of engine cowling frightens them. Remember - center line is always in sight. I suppose you could verify that you will not have a tail strike if you were to jack up the nose wheel to obtain this deck angle, step outside and verify that the tail is indeed a fair distance from a strike.

Some pilots feel uncomfortable because they feel they do not have the visual references needed for lateral control. This is normally not an issue as long as the center line is in sight over the engine cowling.

The proper amount of deck angle ensures you are reaching and holding the highest possible angle of attack into the touchdown without doing a tail strike. This is important because it tends to minimize runway length requirements for the landing and it is nice to the aircraft because you are milking as much lift out of a very slow aircraft for as long as you can which means nice gentle wheel contact at the lowest possible speed. The proper amount of deck angle is part of what leads to the "greased" landing. A greased landing is nothing more than a landing where the pilot has kept as much lift as possible on the wings until the moment of touch down leading to a smoother transfer of aircraft weight from the wings onto the mains.

Another common problem in landings is concrete legs during cross wind landings. I will save this topic for another time. Meantime, get out there and work on keeping proper deck angle while landing when there is no wind or the wind is down the runway. Be safe and be nice to your airplane - even if you are a renter.

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