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Trimmed Stalls

Some years ago I was deposed as an expert witness in the case of an aircraft accident. The aircraft manufacturer was being sued over a fatal accident where the airplane's cabin door had popped open just after takeoff and the airplane stalled.

Some years ago I was deposed as an expert witness in the case of an aircraft accident. The aircraft manufacturer was being sued over a fatal accident where the airplane's cabin door had popped open just after takeoff and the airplane stalled.

Later I learned that the case was not exactly as portrayed -- yes, the cabin door did open, but the pilot controlled the airplane and brought it around for a successful landing. It was only after the pilot landed and properly latched the door did the airplane pitch up sharply and stalled on the second departure.

The case never made it to court, but we can learn from the mishap. It illustrates something I like to teach, especially in airplanes with a wide center-of-gravity loading range. It's what I call the "trimmed stall."

TAKEOFF TRIM SETTING AND LANDING
Every airplane with a pitch trim control (and that's just about everything built since the 1920s) has a safe range of trim settings for takeoff. It's usually marked and sometimes placarded on the panel near the pitch control; airplanes with Pilots Operating Handbooks will mention the safe range of takeoff trim settings. In airplanes with more than two side-by-side seats the takeoff setting is sometimes a range of positions depending on whether aft seats and/or baggage bins are occupied. The takeoff setting usually approximates the trim force required to achieve Vx (best angle of climb) airspeed at full power -- set the trim and you won't have to fight the airplane's tendencies in those critical seconds just after liftoff. Most light training airplanes end up very close to a takeoff trim setting when trimmed for landing. Move into anything much heavier than a Cessna 172, though, and you'll likely note a difference between takeoff and landing trim settings

Danger: Have you ever noticed where your trim is set after landing? If you trim during your final approach and landing flare, the trim may end up somewhere quite different than the takeoff position. In many airplanes, especially those with larger, turbocharged engines, and/or in multiengine airplanes, the "landing" trim may be very nose-high.

INSIDER'S NOTE: I often instruct in turbocharged Beech Bonanzas. With the weight of all that turbocharging equipment (60 pounds or more) ahead of the firewall, and in a typical training configuration (i.e., two people in the front seats, no one in back), the "landing" trim setting often ends up as high as 21 degrees nose UP. Compare that with the takeoff trim setting of only 6 degrees UP in the same configuration; it can be an eye opener.

THE GO AROUND
What'll happen if you're trimmed for approach in this way, and conditions suddenly require a go-around? Since a stable airplane will attempt to seek its trimmed airspeed, as prop blast increases over the tail (resulting from your power increase in the go-around) the trim "thinks" the airplane's flying faster; applying power will cause the nose to pitch up as the airplane seeks to return to its trimmed airspeed.

What happens: Unfortunately, the airplane doesn't "know" exactly how much to pitch up to maintain trimmed speed, and will often over-rotate, or pitch up too much in its recovery. The nose raises a little too high; airspeed falls slightly below trimmed speed. Now too little air is flowing over the tail and the nose drops to regain speed. Left unchecked, the airplane will oscillate up and down several times before it's established on trimmed speed in the climb.

Imagine yours is an airplane that trims very nose-high (like that turbo Bonanza) on final approach. Initiate a go-around and such an airplane will nose up sharply and, if the pilot doesn't prevent it, it may get dangerously close to a stall -- or lose lift entirely. In such airplanes it takes a hefty push forward on the controls to safely go-around from a "landing" trim position, and avoid the "trimmed stall."

PRACTICE THE TRIMMED STALL
To showcase this tendency and the somewhat unexpected control input required to recover, in checkouts and recurrent training I demonstrate and practice stalls a little differently than those in most training and checkrides. Remember: in the Practical Test Standards for pilot certificates and ratings, one of the measures of a successful stall demonstration is that the pilot maintains altitude throughout the maneuver. The FAA has its reasons for this, but I prefer to show stalls and recoveries in a more real-world context.

Strategy: Climb to a safe altitude and perform the required clearing turns to check for traffic. Establish the airplane in an approach configuration, i.e., the airspeed and power setting used on the downwind leg of a visual traffic pattern. Parallel a road or other ground reference that will serve to simulate the runway. Once opposite your simulated touchdown point, change configuration (extend landing gear in retractables; flaps and power as appropriate to your airplane) just like you're flying a VFR pattern. Continue the simulation through the turns to base and final, slowing and trimming the airplane as if ready to land. Allow the normal traffic pattern rate of descent and airspeeds; trim off the pressure just like the "real thing." On your final approach (still at a safe altitude for stall practice) lower the last of the flaps and slow to "book" final approach ("over-the-fence") airspeed. Trim off the remaining pitch pressure.

Tip: Employ a CFI knowledgeable in your make and model of airplane to run this exercise with you.

Now pull the nose slightly upward, to about level with or just above the horizon. Depending on the power you're still carrying, the airplane will decelerate rapidly and may stall almost right away. Recover in the standard manner and see just how much the nose tends to rise with power application. A very powerful engine or a pair of engines without counter-rotating props may yaw rapidly in the recovery also. This is how your airplane will behave in a go-around from a "trimmed" landing condition, or if wind shear or inattention causes a stall on final approach. It's not unusual for my flight student clients to have a "secondary stall," a deepening of the stalled condition, the first time they try a realistic, "trimmed stall."

MODIFYING THE STALL RECOVERY TECHNIQUE
There is a single pitch attitude that provides best angle-of-climb airspeed in a takeoff configuration (the speed "takeoff" trim is designed to achieve). Know this attitude and you'll know how to avoid the trimmed stall. Climb back to altitude and fly your simulated pattern again.

  • This time, on "final approach," simulate a go-around from the landing trim setting. Again you'll see the airplane's tendency to pitch extremely high if you don't properly control its pitch.
  • Let the nose come up, but only to the Vx pitch attitude.
  • HOLD it there with forward control pressure as needed; begin rolling in nose-down trim to relieve the pressure as soon as you've confirmed proper climb configuration (gear, flaps, etc.) and performance.

Mix it up a bit. Try the stall with partial flaps, or during a simulated turn to final. Vary the amount of power to see the different effect. Practice until you're comfortable with stall recoveries where they matter most -- close to the ground in the traffic pattern, and a go-around.

THE BOTTOM LINE: I think the "trimmed stall" is a likely contributor to the crash of that Bonanza about which I testified. The pilot may not have reset the trim for takeoff after the distraction of the open-door-induced return-to-land. Many pilots practice stalls only in preparation for checkrides, then never practice such "uncomfortable" procedures again. Work to avoid "trimmed stalls." Find a CFI and practice until you know the tendency of your airplane in a realistic stall recovery, especially in the traffic pattern.

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About This Author:
Tom Turner is a widely published author and regular forum speaker at EAA's Oshkosh/Airventure and American Bonanza Society. Tom holds an M.S. in Aviation Safety with an emphasis on pilot training methods and human factors. He has worked as lead instructor at FlightSafety International, developed and conducted flight test profiles for modified aircraft and authored three books including: Cockpit Resource Management: The Private Pilot's Guide and Instrument Flying Handbook (both from McGraw-Hill). His flight experience currently spans 3000 hours with approximately 1800 logged as an instructor. Tom's certificate currently shows ATP MEL with Commercial/Instrument privileges in SEL airplanes.
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