Unload for Control Part 1

Using the AOA Tone when the Engine Quits

To control an airplane, the AOA must be below stall.  If we stall unintentionally, we have lost control of the airplane—it’s no longer is going where we want it to.  If we approach a stall, we have to reduce AOA (“unload”) to maintain aircraft control. The airplane stalls at critical AOA, and critical AOA is always the same.  Stall speed will vary with G load (bank angle), gross weight and density altitude.  This is why AOA is such a good reference for maintaining aircraft control when maneuvering near the aerodynamic (stall) limit of the airplane.  Critical AOA, L/Dmax and ONSPEED AOA are always the same for a given flap setting.   The AOA tone allows the pilot to easily hear each of these conditions as well as the transition between them.  To maintain aircraft control at low altitude, we not only have to avoid stalling, we also have to maintain sufficient energy to maneuver.  Maintaining an ONSPEED condition ensures energy is available and turn performance is optimized.    

Energy Management (EM) Review

Energy is power converted into airspeed and altitude.  In terms of energy, ONSPEED AOA is a critical point:  if we pull any slower than ONSPEED, we are expending energy for no net gain in performance.  This simple model is depicted in Figure 1.

Figure 1

If the engine quits, then there is no longer any power being added to our energy equation.  In this case, EM is all about trading altitude and airspeed and arriving over the touchdown zone ONSPEED.  Pitch is the primary control we use to accomplish this, and using AOA allows us to do this as efficiently as possible. The two key AOA’s for EM when engine-out are L/Dmax AOA for maximum range glide and ONSPEED for maximum endurance glide, approach and landing. These AOA’s aren’t affected by gross weight, G load (bank angle) or density altitude—they are always the same. If the airplane has a properly calibrated AOA system and the pilot can easily discern these two key AOA’s, EM becomes fairly straight forward when gliding, power-off.   If we modify the model in Figure 1 to eliminate power, we end up with Figure 2:  the ONSPEED push/pull matrix.  The Goldilocks (just right) zone for gliding flight is between ONSPEED and L/Dmax.

Figure 2
Unload

We “unload” using pitch control to reduce AOA, it’s simply a substitute for the word “push,” so we’ll use those terms interchangeably.  How much we unload depends on what we are trying to accomplish.  In an extreme situation, stall recovery, we want to smoothly unload sufficiently to break the stall by reducing AOA (usually applying sufficient forward pressure to reduce G to less than 1).  Most of the time, unloading is more a matter of “easing” the stick or the yoke forward to relax pitch to maintain our margin above stall. This sort of small, smooth control application is called a “low gain” input.  A “high gain” input is slamming the stick forward.  A smooth pilot makes multiple low gain pitch corrections vs trying to make one perfect correction.  

Unloaded Gliding Turns

One of the easiest ways to visualize the push/pull model is to use the example of gliding flight.  We know that maximum range glide occurs at L/Dmax AOA and maximum endurance glide occurs at ONSPEED AOA. If the engine quits, we want to maintain one of these conditions.  With the power at idle (or the engine failed), every time we bank when gliding, we have to reduce back pressure to maintain angle of attack.  We can bank as steeply as we want, as long as we unload sufficiently to maintain the desired AOA.  When we do this, IAS and descent rate (in feet per minute) are going to increase. Frankly, we don’t care how much, because we are referencing AOA.  If you bank during a glide and don’t reduce back pressure, you risk stalling and losing control of the airplane. If the objective is to stay aloft as long as possible, then the pilot should maintain ONSPEED during the glide, including turns.  Using the aural AOA logic, if the pilot begins a bank without reducing back pressure, they will receive a slow tone, reminding them to ease the back pressure on the stick.  Conversely, when the pilot rolls out of a turn, the tone will transition to a fast cue, reminding the pilot to increase back pressure to maintain ONSPEED.  If maneuvering room is critical, then the pilot should maintain ONSPEED throughout the turn, because best sustained turn performance occurs ONSPEED. This video of an ONSPEED gliding turns illustrates the concept.  The same control technique applies to an L/Dmax (maximum range) glide.  In both videos, you’ll note some AOA excursions (both fast and slow). For human-actuated flight controls, this is perfectly normal—it’s the less-than-perfect pilot making a series of low-gain corrections to attempt to keep AOA as stable as practical while changing bank angle.  The objective is to know when to push or pull--the art is how much, which is simplified based on feedback from the tone.      

Figure 3
Simulated "Flame Out" Practice

Now that we understand how to properly manage AOA in a glide and during gliding turns, we can apply those principles to engine-out landing practice.  A “flame out” occurs when the flame in the cylinder or combustion chamber has gone out, and the engine no longer has the ability to produce power.  When the engine quits, we no longer have the ability to addenergy, only expend it while flying to the crash.  By regularly practicing good AOA management and simulated flame-out techniques, we can develop a good feel for the airplane as well as deriving some known energy “hoops” (called High Key and Low Key) to fly the airplane through when power-off. Exact parameters and configurations will vary from airplane to airplane and will require experimentation to determine optimum parameters for each type.  A basic emergency visual pattern for a typical EAB light aircraft is shown in Figure 3.  The key parameter is to know how much altitude the airplane loses in a 180 degree descending turn at L/Dmax and ONSPEED and adjust AGL altitudes appropriately.  It’s important to accommodate wind conditions, and this is best done by adjusting the low key position. This is depicted in Figure 4.      

Figure 4

In Figure 4, low key “A” represents a nominal, no-wind position.  If there is an undershooting wind (wind component that blows the airplane away from the runway and undershoot final), then low key is adjusted towards position “B.”  A tighter low key is also appropriate for a strong headwind.  If the wind is overshooting (a component that blows the airplane toward the runway and would cause the airplane to overshoot final if not properly compensated for), then low key should be adjusted towards position “C.”

180-Degree Power Off Approach and Landing

The 180 power-off approach and landing is a basic skill taught to all pilots that learn to fly in a single engine airplane.  Techniques vary and there is plenty of excellent discussion in flight training literature; so, we’ll just look at an AOA-based, caveman-simple technique that uses the basic visual landing pattern depicted in Figure 5. The entire pattern is flown ONSPEED with landing flaps set.  All the pilot does is manage AOA and ground track to get the airplane to the TDZ ONSPEED.  

It’s a good idea to keep a small amount of extra energy “in the bank” by being slightly high and/or fast and making several small corrections after you are sure it’s necessary. A correction could be a momentary slip, and/or adjusting the ground track.  Several small, low-gain corrections are better than one big, high-gain attempt.  In Figure 5, the blue line depicts a nominal base turn.  If there is any doubt about the ability to reach the runway, then it’s appropriate to make an immediate bid for the TDZ, depicted by the red dashed line, and consider reducing flaps to a “lift” setting (usually half flaps or less in most aircraft).  If the airplane is high, then squaring off the base turn a bit, as shown by the green line in Figure 5, is appropriate.  A slip or flap adjustment can be applied at any time.  If the airplane is ONSPEED at low key, it has sufficient energy to make the TDZ if the pilot properly flies the base and final. The objective is to arrive over the TDZ ONSPEED.  A 10-12 second final approach helps the pilot achieve stable parameters and alignment for landing.  One other technique to consider is ALWAYS flying the basic, 180 power-off approach (as traffic permits) every time you land.  This allows you constant practice adjusting your base turn for wind conditions and allows you to develop a good sight picture.  An engine in idle produces residual power, so actual engine-out glide characteristics will be slightly different; but shy of shutting the engine down at altitude, practicing in idle power is the best we can safely do on a regular basis. 

Putting It All Together

Now that we understand how to use the AOA tone to assist with flying gliding turns and a basic 180 power-off approach, we can add some additional maneuvering to simulated flame out practice.  The first step is to fly the basic emergency pattern depicted in Figure 3.  Arrive 1250-1500’ AGL over the TDZ at L/DMAX AOA and slow to ONSPEED AOA while descending to “wind adjusted” low key for a 180 power-off approach.  After the basic emergency pattern is mastered, we can add additional EM practice by flying SFO Pattern A and B.       

SFO Pattern A

The SFO Pattern A is designed for all pilots, including students.  It combines elements of L/Dmax glide with ONSPEED glide and ends in a conventional 180opower-off approach (Figure 6).  It only requires medium bank angles or less, and the pilot must adjust bank angle during the descent to accommodate winds (much like a turn around a point).  

SFO Pattern B

The SFO Pattern B combines elements of the 1080 steep-spiral taught to commercial pilots and flight instructors with a standard 180 power-off approach.   Like the Pattern A, it requires diligent energy management to perform the entire maneuver and arrive over the touchdown zone ONSPEED.   During the 1080 steep spiral, it’s necessary to adjust bank angle/G/AOA to accommodate wind conditions.  Under no-wind conditions, a constant ONSPEED spiral at about 2 G’s will work well, but if there is any wind aloft, it will be necessary to vary AOA from ONSPEED when turning down wind to L/Dmax when turning up wind.  The aural AOA logic makes this relatively simple.  This pattern requires fairly steep bank angles (up to 60 degrees) and should only be practiced by experienced pilots or under the supervision of an instructor.  Interestingly, due to the high turn rate, less altitude is lost in each descending 360 than would be the case with a gently banked ONSPEED turn, so the initial entry altitude will be lower than SFO Pattern A.  SFO Pattern B is shown in Figure 7.

Emergency Turn-back after Takeoff

Another use of ONSPEED is to provide optimum turn performance, which means best sustained turn rate and smallest sustained turn radius. If the engine fails at low altitude after takeoff, maneuvering options are limited. ONSPEED provides a simple performance cue that results in optimum turn performance, maximum endurance glide and optimum energy for approach and landing. In other words, if the engine quits on takeoff, maintain ONSPEED and fly the airplane to the crash. In addition to having sufficient altitude the pilot must also consider whether or not there is sufficient turning room to get the airplane back to the runway and whether or not there is enough runway to slow the airplane to ONSPEED for touchdown. Because we normally takeoff into the wind, any emergency turn back will also be complicated by having to land with a tailwind. There is no one, correct technique to apply in this situation; so I’ll offer the following decision matrix if power is lost during the initial climb segment: 1) adjust pitch to maintain ONSPEED; 2) Apply “lift” flaps (usually1⁄2 flaps or less); 3) Fly the airplane—no slower than ONSPEED all the way to the flare; 4) Decide: Where are you going? Where is your turning room?

Figure 8 depicts an emergency turn-back scenario after takeoff. At position A, the engine fails and the pilot establishes an ONSPEED condition and applies lift flaps while beginning a direct turn back to the desired TDZ. The turn direction should be in the direction of any crosswind component. This reduces turn radius relative to the ground and provides more energy (altitude) than a downwind turn. ONSPEED is maintained throughout the maneuver. Once pointed back at the airfield, an ONSPEED (or faster) reposition maneuver is flown to align the airplane with the runway. As soon as the airplane is pointed at the runway, the pilot has to assess energy:

1. If high and/or fast, then landing flaps, a slip or both are appropriate. In this high energy scenario, after runway alignment, it may be necessary to aerobrake to assist with slowing the airplane down during the transition to landing. This is accomplished with a forward slip which increases aerodynamic drag (D). Once ONSPEED is achieved, a normal landing is accomplished. The need for maximum braking should be anticipated. Depending on touchdown speed and runway remaining, an intentional ground loop may be preferable to running off the end of the runway if the airplane cannot be stopped on the landing surface with maximum braking.

2. If low and/or slow, adjust pitch to maintain ONSPEED and assess the ability to reposition to the runway. It may be necessary to roll-out or maneuver for an off-field touchdown. If the desired touchdown zone is rising in the windscreen and the airplane is ONSPEED (or slow), you will touchdown short. A low energy scenario is best avoided by having a minimum altitude from which a turn back is attempted. This altitude is airplane and pilot dependent and varies. It can only be determined by practice. If there is any doubt about having sufficient energy to reach the runway, the initial maneuver should simply be a turn into the wind, while slowing to ONSPEED (this is depicted by the red dashed line in Figure 8). Turning into any crosswind component effectively reduces turn radius.

Summary

1.  If the engine quits, maintain ONSPEED or L/Dmax AOA.

2.  Unload (ease back pressure or push) to maintain aircraft control when banking the airplane in a glide.

3.  Practice!  Consider flying 180 power off approach on a regular basis. Learn the proper AGL parameters for high and low key for your airplane.

4.  If the engine fails on takeoff: Fly ONSPEED and apply lift flaps. No slower than ONSPEED until the flare. Decide.  

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