Safety Pilot:

Balance of power

October 1, 2010

Some fairly experienced fliers have forgotten that high-powered engines driving propellers can cause aircraft to do unhelpful things during takeoff, balked landings, and stalls.

In the past five years there have been more than 100 accidents where too much power applied too quickly at too high an angle of attack with too little rudder applied too slowly resulted in a too-bad outcome.

Prop rotation is a byproduct of torque. As the internal parts of the engine and prop rotate clockwise (when viewed from the rear), the torque effect will rotate the aircraft to the left, or counterclockwise. On the ground the landing gear prevents the aircraft from moving much. This effect is clearly seen when revving your GTO’s 389 mill at a stoplight. The car will rock slightly to the left. When airborne, the aircraft will also roll left, although the manufacturer usually offsets the left wing slightly to produce more lift to counteract the roll. In a climb attitude, the descending prop blade (on the right side) produces more thrust, which adds to the left turn—“P-factor” is most noticeable at high power and high angles of attack.

Next, we have prop slipstream effect from the air that corkscrews over the top of the fuselage and aft, striking the vertical stabilizer on the left side—pushing the tail right and the nose left. The tightness of the corkscrew varies with airspeed. At cruise the effect is much less because the tail has moved ahead of most of the spiral, diminishing the effect. In some aircraft the stabilizer is offset to counteract this.

Finally, for tailwheel aircraft, the gyroscopic precession caused by the change in the plane of rotation of the prop when the tail is raised for takeoff also causes the aircraft to turn left. The bigger the engine, the more pronounced this will be. That’s a lot of aerodynamics to internalize, so let’s keep it simple. Just remember that all of this causes the aircraft to turn left. (Disclaimer: Twins with counterrotating props; jets; and certain contrarotating engines do not behave this way.) Right hand (applying power) also means right foot applying counteracting rudder. (There’s always an exception—most stick-controlled aircraft require the left hand for throttle.) Smooth and steady power application is essential. Abrupt movements beget abrupt response, which can make things really exciting. The bigger the engine and the slower the aircraft is moving, the larger the left turn, the more pronounced the rolling tendencies—and the more excitement generated for both the pilot and onlookers.

In stalls, the addition of power is destabilizing if applied too quickly and without appropriate right rudder. As an instructor, I got dumped out of numerous takeoff and departure stalls by students in low-powered trainers when they failed to apply appropriate right rudder, even though we had discussed it before the maneuver. The left wing drops and the aircraft rolls left fairly abruptly. Emphatic right rudder also could make things way too interesting.

In reduced-power situations such as an approach-to-landing stall, everything is quite comfortable unless rapid power application is made before the stall is broken. Yaw is introduced at just the right time to produce a potential power spin.

A Cirrus SR20 accident appears to illustrate this by the introduction of too much power too soon. The electronic primary flight display and flight management system recorded the sequence. The commercial pilot with a CFI on board set up for what looked like a partial power-on stall (1,840 rpm) about 3,000 feet agl. The aircraft stalled at 60 knots. According to the NTSB, “The engine rpm decreased to 1,050 rpm at 1417:28, and the airplane was heading 081 degrees at 50 knots indicated airspeed and groundspeed.” One second later the airplane rolled 13 degrees to the left before rolling 28 degrees right. It then reversed back to the left. “At 1417:34, the airplane was at 3,131 feet heading 064 degrees, the engine rpm had increased to 2,500 rpm, the indicated airspeed was 54 knots, and the groundspeed was 52 knots. A left spin began at 1417:35, and the recorded primary flight display data ends at 1418:02.” The airframe parachute deployed at approximated 200 feet agl but failed to arrest the descent before ground impact. The Cirrus POH is very clear in specifying that if a spin develops, immediately deploy the chute.

S-turns on final can be bad. Slow, turning flight close to the ground; distraction from a preceding aircraft; and a nose-down pitch attitude in landing configuration may not look or feel like typical practice stalls—but the envelope’s edge is close. A Socata TBM following a slower aircraft on final was asked by the tower to S-turn to provide spacing. Witnesses saw the airplane wing over onto its back and go straight down. The pilot probably made a quick power application because the flight was close to the ground. Get too far into a corner and there may not be a way out.

Balked landings or missed approaches also are popular areas for the radical left turn to assert itself. Aircraft seldom depart the right side of the runway under high power. In flight, as in politics, balanced and smoothly applied power is just what’s needed.

AOPA Summit

AOPA Foundation President Bruce Landsberg is a featured speaker at AOPA Aviation Summit 2010, November 11 through 13 in Long Beach, California. He will present “What Went Wrong?” on November 12. Visit the website for more information.