January 1, 2008
We all strive for precision and safety in our flying. If there is a recurring theme to smoothly lifting off into climb, maneuvering with a safe margin above stall, squeaking the tires onto the runway at our planned touchdown point, and maintaining control and precision even in emergencies, that theme is airspeed control.
Airspeed is a result of power, aircraft attitude and configuration (position of drag-producing devices like flaps). For a given combination of power, pitch and flaps, there will be a single resulting stable airspeed. More correctly, a combination results in a specific angle of attack (AoA), which in turn determines aircraft performance. Angle of the attack is the measured angle between the chord line of the wing and the relative wind. More simply, as Wolfgang Langewiesche put it in the timeless classic Stick and Rudder, "... the angle at which the wing meets the air." AoA drives virtually all aircraft control and performance.
I Warned You
|Angle of Attack Indicator
Many military aircraft and business jets have versions of an AoA meter and there is at least one uncertified AoA-sensing device available for experimental aircraft. Stall warning systems -- usually a tab or vane that deflects in the air flow on the leading edge of the wing that signals with a buzzer or light in the cockpit -- are usually thought to go off at an airspeed a few knots above stall. In fact they are direct AoA sensors that sound/flash the warning a few degrees below stalling angle of attack: a true AoA sensor for a small range of angles of attack.
But very few of us have visual AoA indicators unless our airplane is so equipped. So we must accept indicated airspeed as an indirect measure of AoA. Control airspeed and you control safety and performance.
Controlling airspeed has a significant role in safety of flight. We spend a lot of time concentrating on airspeed control in the context of stalls and flying too slowly, but there are times that too great an airspeed can be an equally hazardous condition. Take these lessons extrapolated from a series of recent (and common) aircraft mishap reports:
- Improper airspeed control may contribute to blown tires and/or loss of directional control on landing. When you do touch down, you'll be tempted to brake excessively and risk blowing a tire. If a tire blows, you may not be able to control the direction of rollout. In extreme cases the airplane collides with an object or cartwheels after it leaves the runway ... all because of a few extra knots on final approach.
- You may land hard by trying to "force" the airplane down where you want it despite too great an airspeed, damaging the landing gear on impact. Or you may generate a dangerous "wheelbarrow" (loss of steering from too much weight on a nosewheel). More dangerously, you may "porpoise" (up and down pitching as a result of loss of pitch control or pilot-induced oscillations trying to correct), which can easily lead to landing-gear, tire or propeller-strike damage.
- Excessive speed on final approach may cause you to overshoot the landing zone entirely, and if you elect not to go around for a second attempt, you risk running off the end of the runway, possibly with enough residual energy (speed) to cause damage or injury.
- Excessive airspeed may also be a symptom of a configuration error. If you have the proper settings for power, attitude and vertical speed on final approach and your airspeed is excessive, the first place to look to resolve the discrepancy (in retractable-gear airplanes) is the gear-position indicators. In other airplanes, it might be that flaps aren't set where you think they are, and you have a smaller stall margin. Airspeed may be a root cause of mishaps; airspeed may also be a symptom of some other condition like an imminent gear-up landing or mis-set flaps. Don't treat the symptom (by adjusting pitch for airspeed); look for and treat the cause (failure to meet power, pitch and configuration targets).
Besides the obvious stall if airspeed gets too low for the airplane's G-loading (AoA becomes too great), flying within the AoA envelope but slower than optimum creates a big increase in drag, and this in turn reduces performance. Climb at too slow a speed (trying to clear an obstacle or through a gap between clouds, for instance) and the rate of climb will actually be worse. Come in to land below proper final-approach speed and excess drag brings the airplane down at a steeper angle. You may find yourself in the confusing situation where you need to push the nose forward to clear an obstacle, if your speed (AoA) is too great. So in addition to the threat of a stall, reduced airspeed on climbout or landing can cause you to miss performance goals and present a safety hazard.
Strong Or Gusty Winds
Strong or gusty winds can alter the AoA of a landing or departing airplane. I define "strong" as 15 knots or greater, but it's really a matter of the specific airplane's capability and your currency in windy flight. Talk to your instructor to evaluate what defines "strong" wind for you.
Give yourself a greater margin above stalling AoA when taking off or landing in strong or gusty winds. As we've said, there's a hazard of porpoising or wheelbarrowing if rolling on the runway at too great of speed, so establishing this margin on takeoff or using it on landing takes some finesse. I wouldn't delay rotation beyond the "book" takeoff speed, but I also would not use the normal piston airplane technique of easing the nose up at a lower speed and letting it lift off "when it's ready." I also would (and have in training many times) impose a personal limitation against soft- or short-field takeoff technique in strong or gusty winds The mishap record reveals a history of lifting off at speeds below which the airplane (or pilot) can adequately compensate for crosswinds, or rolls resulting from low-level turbulence.
So ... power up, accelerate to liftoff speed (from the Pilots Operating Handbook Takeoff Performance chart or other reference), then bring the nose up deliberately to a lift-off attitude. In most airplanes a little aft pressure on the elevator controls will prevent shimmy or wheelbarrowing as the airplane accelerates, but still provides enough traction to assist in steering and directional control until the flight controls are fully effective.
Normally, you'll "rotate" to a nose-up attitude that results in a VY or VX attitude. In many light airplanes it takes from seven- to 10-degrees of nose up attitude (using the attitude indicator) to hit the VY/VX range. In strong or gusty winds, bring it to about five degrees nose up, or a little less than whatever is normal for your aircraft. This gives you an added cushion below stalling angle of attack, in case of turbulence.
On landing, carry a little extra power and land "flatter," or at a shallower angle. This is where the usual "half the gust factor" recommendation for airspeed increase in gusty winds comes from. The idea again is to maintain a lower angle of attack for stall protection.
Takeoff and landing stall avoidance also implies avoidance of true short-field/obstacle-clearance takeoffs and landings in strong or gusty surface winds. The high angles of attack required don't support the need for a greater stall margin as AoA varies rapidly in the gusts. Soft-field techniques, which put the airplane in the air at a very low airspeed on takeoff and keep it there for a long time on landing, are also counterproductive to stall margins and directional control in strong winds or gusts. Use longer, firm or paved runways in strong or gusty wind conditions, or delay flying until conditions improve.
Airspeed control is essential when dealing with abnormal indications or emergencies. Look at the Emergency Procedures section of your airplane's manual and you'll see that it's all about maintaining precise airspeeds. Fly too slow and control suffers, and you risk a stall. Fly either too fast or too slow and drag builds, reducing available performance. "Best glide" airspeed is actually an indirect measure of the optimum angle of attack for the least amount of drag -- and if the engine quits, this is the speed where the airplane will have the greatest gliding distance.
Almost nowhere is airspeed control more critical than in the event of an engine failure just after takeoff in a piston twin. VMCA is the minimum indicated airspeed at which a multiengine airplane has directional control while airborne in the event of an engine failure under worst-case conditions as identified in the rules for aircraft certification. The same loss-of-control "VMCA effect" exists at lower indicated airspeeds as each of the certification variables change from their most critical conditions. VYSE, or best single-engine rate of climb, is an airspeed approximating the least-drag AoA, and it reduces with the weight of the airplane as well. For any given airplane weight there is a single AoA (referenced as indicated airspeed) that provides maximum available performance with an engine out, with varying but lesser performance available at other AoAs (indicated airspeeds). Control authority is a direct function of air flow over control surfaces, or indicated airspeed (not just AoA). So working to maintain a precise airspeed satisfies both performance criteria to avoid loss of control below VMCA.
Both in single-engine airplanes and twins, airspeed control is the vital element to survival in the event of an engine failure.
Managing angle of attack, indirectly measured as indicated airspeed, is the key to safety and precision in flying. It's all about airspeed. Know the proper airspeeds for normal and emergency flight, and strive to always master airspeed control.
Fly safe, and have fun!
Thomas P. Turner's Leading Edge columns are collected here.