Wednesday, 8 March 2017
WHAT ABOUT THOSE SPINS?
Acknowledgements: AIR FACTS (Abridged from an article by Richard Collins)
There should be no mystery…
On the NTSB’s 2017-2018 most wanted list of transportation safety improvements: “From 2008 to 2014, nearly 48 percent of fatal fixed-wing GA accidents in the United States resulted from pilots losing control of their aircraft in flight.”
The sad thing is that this has been forever true. The low altitude, low speed loss of control has always dominated and this was often dismissed with the comment: “He ran out of airspeed and ideas at the same time ………...”
Do pilots know enough about spins?
Spins used to be part of the private pilot training syllabus, but even after the requirement was dropped most instructors included spin demonstrations before first solo and spin training before stalls solo; because students could accidentally spin while practicing stalls and they needed to know how to recover. In 1953, precision spins were a requirement for an instructor. They were done three ways: straight ahead, out of the bottom of a turn, and out of the top of a turn. The spin out of the top was the most difficult.
· Most pilots usually spin to the left, and out of the bottom of left turns I used about 45 degrees of bank, slowed to the stall in the turn and at the stall gave it full left rudder and full right aileron.
· Out of the top, the same left turn, the same slowing, but a different technique approaching the stall. The stick was brought back smartly as full right rudder was applied. Any other way, and the airplane just wallowed into the spin. I didn’t use full opposite aileron because it didn’t seem a natural thing to do.
In the early 1950s we knew the airplane would spin and we knew how to spin and recover, but we really didn’t know why the airplane would spin. We did know that it took stalling and yawing to make the airplane spin and that the corkscrew motion was caused by the angle of attack on the bottom wing being greater than that of the top (outside) wing. What a lot of pilots didn’t know was that in some airplanes you could get enough adverse yaw out of just the ailerons to cause an airplane to spin. Most pilots thought it took full rudder.
When an airplane is spinning it is stalled, so the airspeed is stable just below the stalling speed. Some airplanes will fly out of a spin and into a spiral, with the airspeed building. This needs to be noted quickly and a recovery started from the dive. When spinning, the nose of some airplanes seems to be quite low, others even lower. Some NASA research found that in a steep (nose-down) spin the angle of attack is 20 to 30 degrees. That is not too far from the stalling angle, which averages about 18, and what this means is that the airplane should recover quickly. In a moderately steep spin it is 30 to 45 degrees which would make recovery slower, and in a flat spin the angle of attack is from 65 to 90 degrees. Flat spins are unrecoverable without a spin chute.
Is your aircraft spin-certified?
The Cessna 172 is approved for spins when operated in the Utility Category. Piper certified at least one model of the Cherokee for spins, to be competitive with Cessna trainers, and designed the Tomahawk to have more active spin characteristics than the Cessna 150/152. Cautionary note: The location of the airplane’s centre of gravity is quite important. The further aft it is, the easier the airplane is to spin but recovery is more difficult. With the CG too far aft, recovery will be impossible. The Cessna P210 has a certified aft CG limit of 52 inches, but most have had to put the limit at 50 inches (in learning this at least two airplanes were lost in unrecoverable spins).
How much altitude is lost in a spin?
Lighter wing loading means a lower stalling speed and correspondingly less altitude loss in a spin. The loss in an accidental spin would include loss in the departure from controlled flight and as the airplane entered the spin. Then loss in the time to recognize what happened, then the loss when breaking the spin, then the loss recovering from the dive after the spin. I’ve seen numbers on this ranging from 1,000 feet to 1,500 feet for airplanes in the PA-28/172 class. What that means is that if you stall below that altitude, all those stalls you practiced at altitude, and any spins you have done up high, won’t do you any good. In the spin itself, each turn might account for 400 to 500 feet of loss but again, the entry and recovery have to be considered. When an airplane departs controlled flight and crashes, witnesses on the ground often say “it wobbled and then rolled over and dived into the ground”. If it had been a little higher they would have probably seen the spin start to develop before it hit the ground.
So should we? …..
I always enjoyed spins and think that all pilots would benefit from an exposure to them. That’s the best way to learn that with enough altitude in the right kind of airplane they are fun and relatively benign. But without enough altitude they are a really quick way to snuff out your lights!