Fly The Wing

Each week in his Flying Lessons newsletter, CFI Tom Turner sheds light on another aspect of safe flying. He recently touched on a topic I realized I don’t spend a great deal of time with during primary (Private Pilot) flight training. This is something that is critical during tailwheel instruction --- due to that CG being behind the main wheels and the weathervaning that results --- but we should take the same approach with new students in tricycle gear aircraft too. The subject is bounces during landing, and multiple bounces, better known as PIOs. Here’s Tom’s notes, followed by a few of my own.

A Pilot-Induced Oscillation, or PIO, usually results when a pilot allows the aircraft to land hard and then, in an attempt to recover from the bounce, he lags just enough behind the aircraft's movements that the pilot's control inputs add to the severity of a movement up or down.

This in turn prompts the pilot to add additional pitch inputs which, if the pilot continues to “chase” the airplane’s movements, only makes matters worse. A common outcome is a propeller strike (in prop-driven aircraft), or a hard tail strike that then drives the nose downward and causes a propeller strike in the next pitch oscillation. Another common outcome is a hard impact on the airplane’s nose gear, followed by nose gear collapse.

Ground effect decreases elevator control effectiveness and increases the effort required to raise the nose. Not enough elevator or stabilator trim can result in a nose-low contact with the runway and a porpoise develops.

To prevent PIO, pay careful attention to airplane speed during final approach and the flare.

If you bounce and a PIO begins to develop, your best action is to attain and hold a shallow climb pitch attitude as you add power and go around. Once in a PIO event it's virtually impossible to dampen out the pitch excursions and continue the landing--and history shows it's not worth trying.

I teach and follow a one-bounce, two-bounce rule. If I bounce the landing and cannot establish the correct pitch attitude to re-flare and set the airplane down smoothly, I’ll immediately establish the go-around attitude as I add power. If I think I’ve got the right attitude but bounce the landing a second time, I’ll go around without any further thought.

Good tips, Tom. I realized while reading this that I have not seen a student get into PIOs in some time; maybe in years. Couple reasons might be because I constantly harp about nailing airspeed on final, transitioning to an “outside-the-airplane” sight picture on short final and never landing on the nosewheel. But it’s something I should probably begin to induce, just so students know what a PIO looks like and how to recover from it (go around).

If you always land on 27-Right at Gillespie in calm or mild winds in the same airplane, you could go years and never bounce a landing or get into PIO territory. But here are a few of the contributing factors that could result in bouncing:

•Landing at an unfamiliar airport and not flying the same stabilized approach (letting airspeed on final get too high);
•Landing with a strong headwind and flying too fast (remember your ground speed will be much lower with a strong headwind, but you should fly the same indicated airspeed);
•Landing on a wider than usual runway, creating the risk of leveling off too high, landing hard and bouncing (use the visual aiming point method and nail airspeed on final);
•Downsloping runway or terrain creates the same scenario as a wider runway (above);
•Extremely clear air (bright conditions such as an airport at high altitude), can also result in a high and fast approach;
•Landing at any airport in an unfamiliar airplane. You should already have practiced slow flight/MCA and at least Power-off stalls in checking out a new airplane, so you know what to expect when you’re landing.

Remember, the airplane is not literally ‘bouncing’ like a rubber ball; what is happening is you still have too much airspeed,

coupled with an angle of attack that enables the airplane to fly off the runway again. To bystanders it looks like it ‘bounced’, but it merely decided it wasn’t done flying --- there was still a bit of lift being generated by the wings, so it took off again. No, there’s not enough lift to climb to 50’, but there is just enough energy to climb a few feet, then stall, return to the ground again, perhaps this time with even more of a THUNK! Or, if you’re really not in control of the airplane and you allow the nose to drop, the nosewheel slams onto the runway first, followed by a prop strike shortly thereafter.

In Aerodynamics for Naval Aviators, the author writes, “When the pilot induced oscillation is encountered, the most effective solution is an immediate release of the controls.” Maybe not great advice close to the ground, but keep in mind that ‘out-of-sync’ control inputs are capable of producing damaging flight loads (> +3.8/-1.52 G’s) and loss of control of the airplane.

Get out of your comfort zone; fly into many different airports in different airplanes and work to nail approach and landing speeds and purge bounces and PIOs from your repertoire.

Here’s a record from the NTSB files of what happened when a Student Pilot in Santa Ynez tried to fight the PIO’s and lost...

The student pilot was conducting his final required solo cross-country flight. The flight to the first destination was uneventful, and the pilot entered the left-hand traffic pattern for the runway at the non-towered airport. The pilot reported that his base-to-final turn was slightly "wide," which placed the airplane to the right of the extended runway centerline. He also reported that he was "a little high" on the final approach. He corrected his flight path, and was satisfied that the airplane was positioned for a normal landing. The pilot used full flaps for this landing, as was his normal habit pattern. The airplane landed slightly beyond where the pilot had planned, but not far enough down the runway to cause him any concern. The touchdown was "hard," the airplane bounced, and the pilot "re-flared" to correct the situation. The airplane began "porpoising" and bounced at least two more times before the pilot was able to regain control and keep it on the ground. During the bounces, the pilot did not adjust the power or consider aborting the landing attempt. Although he had learned about porpoising in his ground training, he had never experienced it prior to this event, and did not immediately recall or implement the flight school's recommended corrective action, which was a go-around. The pilot taxied the airplane to a parking spot, and discovered that the airplane was substantially damaged. The firewall, cockpit floor, and several fuselage skins were buckled, and elevator travel was restricted.

The pilot reported no preimpact mechanical malfunctions or failures with the airplane that would have precluded normal operation.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

The student pilot's inadequate flare and loss of aircraft control during landing, which resulted in multiple bounces.

10/26/13

The FAA’s Airplane Flying Handbook calls PIO by the archaic name “porpoising,”

a description of the porpoise-like, arching path followed by an airplane in a PIO.