SW Aviator Magazine - August/September 1999 issue
Hangar Flying
Test Flying Your Aircraft
by Karry Ray
"The object of the game, is not to cheat death. The object is not to let him play!"

Frequently in aviation, each of us must be trusting of someone else. And yet we must continue to check and cross-check against that trust on the small chance that the other person has done what we all have at some time: goofed without realizing it.

Each time a pilot advances the throttle levers upon takeoff, he (or she) puts his trust—and that of the passengers—in the skill and judgment of the mechanics who signed off the aircraft as being in safe operating condition. And those mechanics have placed the same kind of trust in the component specialists who overhauled the part, bench tested and tagged it as serviceable, then placed it into stock. Thus, the line of trust goes on.

"At the worst possible time, the worst possible thing WILL happen."
-Murphy's Law

The very first thing you need to ask yourself is this: Can I verify or troubleshoot the problem without flying the aircraft? If the answer is no, then a test flight is in order, and the preparation begins.

Let's begin with the pilot:

  • Are you legal to fly the aircraft? (flight review current, medical current, recent flight experience in category class or—better yet—make and model)
  • Are you healthy? (Head colds, medications, STRESS, scuba diving and other factors may affect your ability to be 100%.)
  • Be really honest with yourself. Do you have the skills required to successfully complete the proposed test flight? If you don 't check your ego, you better check your estate planning; someone's going to need it!

To explain myself, I will use a real life scenario. It was one of a handful of times during my career that I was very glad to see terra firma again!

An owner reported a rough engine in his Rockwell Commander 114TC during descent. He left me with the aircraft and swore he would not fly "that thing" again until it was fixed.

"Test fly it if you need to," he said, and promptly headed off to the casinos. Knowing now what I was about to experience in his aircraft, I'm certain the first place he stopped was the bar.

I was about to violate rules 1 and 3, listed above. Though I had never flown a 114TC before, I was young and dumb and figured, "I've got a pocket full of tickets and 400 hours. No sweat!"

"A superior pilot uses his superior judgment to avoid situations that require the use of his superior skill."

Once you've decided to take the test flight, it's time to plan, plan, plan.

Things to consider before you plan the actual flight:

  • Whether emergency response is available.
  • What the emergency landing sites are...on airport...off airport...on local terrain.
  • Runway length, obstructions and condition (wet, dry, dirt, grass, etc.).
  • Wind, weather, and time of day. (My rule is this: Never test fly an aircraft at night or IFR!)
  • Whether ground crew and communications will be available at the airport.
  • Where you will conduct the test, if anybody knows where you will be, and what time they should expect you back.

Each test flight is different and conducted for a lot of different reasons, so when you begin to plan the actual flight ask yourself a few questions. Is this a troubleshooting flight (by far the most dangerous)? Or is this flight to verify that the problem has been repaired and all systems function normally? These answers will play a large part in planning the actual flight.

Other important considerations for both types of test flights:

  • What do I expect to see during the flight: high/low fuel press, flux oil pressure or temperature, onset of vibration? In other words, if the aircraft had a rough engine on descent, expect to see a smooth running engine on descent. But plan for the engine to get rough.
  • How will the aircraft behave if all goes according to plan?
  • How will it bite me when the plan fails?
  • At what point will I consider the test successful? Conversely, at what point will I terminate the flight if the problem recurs or a new problem crops up?

With the Commander, I tried everything I could think of trying to get it to run rough on the ground. I inspected the mags (or mag in this case: Bendix D3000, single drive, dual magneto), plugs, leads, fuel system, prop, high and low power runs...nothing...smooth as glass.

I was beginning to suspect that the pilot had hit some rough air (common in Vegas in the summer) and mistook the summertime hammering as a rough engine. I told myself, “Well, lets fly it and see.”

I may have been dumb but I sure was not going to be stupid. I got the Pilot Operating Handbook out and studied it for a while. I read through the emergency procedures for fuel, gear, and basic engine limits. I also memorized the best glide speed and climb power settings. But that was about the extent of my test flight preparation—except for opening the book to the emergency procedures section and placing it on the seat next to me.

During the takeoff roll, my eyes were glued to the gauges, looking for the first sign of a problem. All seemed well. Lift off was smooth. “That thing” was flying pretty nicely!

I knew enough to leave the power alone until I could make it back to the airport, just in case. When I got enough altitude to make the field, I pulled the throttle and prop back to a climb setting. It was still smooth, except for the turbulence.

I climbed out to around 10,500 ft. MSL and headed for the best nearby area to do the troubleshooting: just East of Mt. Charleston around Red Rock Park. There was plenty of open space and a nice straight road for landing, if need be.

I adjusted the mixture, then leaned it until the engine got a little rough—nothing unusual there. I richened it back to normal and ran the prop through its paces—nothing unusual there, either. What else could I do to duplicate the problem?

“Oh yeah,” I thought. “The mags. Might as well try them.”

The right mag gave me about a 100 RPM drop—OK for the power setting. The left mag was another story. The craft vibrated, and the world went into a blur. I couldn’t see the instrument panel. I couldn’t grab the mag switch, I couldn’t grab the throttle. Stuff was flying all over the cabin. The POH hit me in the shoulder on its way to the right side of my head. I was never sure if the POH knocked my headset off or if the vibrations did.

After what seemed an eternity (only about 3 or 4 seconds in reality), I finally—after a few fruitless swipes—grabbed the throttle and pulled the power off. The severe vibration died to a moderate level, and the instrument panel was no longer trying to leave the mounts.

I switched back to both mags and the plane smoothed out. It was still a little rough, but at least I could touch the controls I wanted. It was such a relief to have some quiet that it took me a few seconds to realize that it was, as they say in the movies, a little too quiet.

The power was still at idle, and I had been hanging onto the yoke for dear life. The nose was well above the horizon—not good! As the aircraft stalled, I remember thinking, “What next?”

The answer came soon: a spin. Fortunately, the Commander is a fairly straightforward aircraft. As the nose fell back through the horizon, I added power, got the old girl pointed home, and swore I would not touch anything until I was on final.

On the ground, I pulled the mag apart and found that the laser welded contact had broken loose from the distributor gear—which had shed a few teeth and was in danger of taking out the other half of the mag. An AD note had not been complied with during the last Annual. I also inspected the engine and tail areas due to the vibration, changing one Lord mount where the rubber was split.

About the only things I did right during the test flight were leaving the power set after takeoff and not touching anything after I had the rough mag under control. That's a pretty short list!

"Go from the known to the unknown...slowly!"

The Flight Plan:
Decide what parameters are in the safety zone, and determine how to approach the danger zone while leaving yourself an out. Discuss the flight with your mechanic; get ideas on how to identify or verify what the problem may be. Construct the flight so that you are efficient and the order is logical. Minimize power on/off, climbs/descents, etc.

The test flight should be conducted alone. Very few scenarios exist that require two people on board. Minimize the risk.

Notify someone where you are going, what you are doing and when you will be back—and stick to that plan. Don’t head over to Joe Bagodoughnuts’ house for a quick low pass. Return on time.

Write the flight profile on a sheet of paper (leaving enough room for notes. Use a knee board):

  1. All systems indicate published figures on takeoff.
  2. Climb at 90 KTS (or what the book says).
  3. Reduce power at 1,200 ft. AGL (or when I can make the field) to climb setting, note fuel, MP and RPM indications.
  4. Level off at 10,500 MSL; reduce to cruise settings; note fuel, MP and RPM indications.
  5. Airspeed stabilized at 140 indicated (or what the book says).
  6. Clearing turns.
  7. Push nose over and establish dive until within 5 KTS of red line at cruise power.
  8. 8. Regain lost altitude; push nose over at onset of stall warning.
  9. Max power and RPM, level flight; if OK then reduce back to cruise power.
  10. Clearing turns.
  11. Switch fuel tanks; if OK reduce power to 18 inches and 1800 RPM, stabilize, note indicated airspeed, fuel and oil indications.
  12. Check left mag; both; check right mag.
  13. Return to cruise settings and return to the airport; maintain altitude until airport is made.

Use this time to sharpen your skills, hold airspeed to +/- a knot or two, and remain focused on the task at hand. Resist the urge to fiddle with the new GPS. Fly your profile card.

If I were to conduct the Commander flight today, I would have a flight profile similar to the one above. Get in the books and write the figures (fuel flow, MP, RPM and airspeed) for all anticipated configurations: take-off, climb, cruise and reduce power. These will become test points.

If the fuel flow at cruise is too high, check the mixture. Did you lean it? If you did, then it may be a partially plugged fuel injector. Each test point becomes what you expect to see. If it doesn't match, then it may be a problem or something you overlooked.

Resolve the issue before continuing on. Why would I dive the aircraft to within 5 KTS of red line? First, this was the condition as reported by the owner. Second, by going to near red line, and then almost to a stall, I have eliminated vibration/flutter as a possibility within the normal flight regime and returned to near the 10,500 MSL test floor, trading airspeed for altitude.

Be careful when conducting this type of testing; it is very easy to overstress the aircraft near red line speeds. If you suspect airframe vibration/flutter, approach the red line in 5-10 knot increments. Use all of your senses. At the first hint of a vibration, reduce power and pull the nose up gently.

Okay, so the test flight you are conducting is not in the latest unproven aeronautical wonder, requiring exceptional skills, lightning reflexes and a large dose of courage. You can still exhibit the same level of professionalism you would need to pilot the GO-FAST 2000 on its maiden flight. Just remember to conduct in-depth research, engage in thorough preflight planning, and know the limitations—your own and those of the aircraft you intend to fly.

In fact, those are the same things you should be doing for every flight!

Make it right from the ground up.

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The material in this publication is for advisory information only and should not be relied upon for navigation, maintenance or flight techniques. SW Regional Publications and the staff neither assume any responsibilty for the accuracy of this publication's content nor any liability arising out of it. Fly safe.