22 5/24/95AC 90-89A SECTION 11. ADDITIONAL ENGINE TESTS ‘‘Always go with the best fix not the cheapest fix.’’ Bill Deeth, Master Mechanic (1994)
1. OBJECTIVE. To determine if the engine supply of fuel is adequate at all angles of attack.
a. Mixture and Idle Speed Check: After completing the initial engine ‘‘run-in’’ tests, check the idle speed and mixture settings. To determine if the mixture setting is correct, perform the following:
(1) Warm up the engine until all readings are normal
(2) Adjust the engine rpm to the recommended idle rpm
(3) Slowly pull the mixture control back to idle cut-off
(4) Just before the engine quits, the engine rpm should rise about 50 rpm if the mixture is properly adjusted. If the rpm drops off without any increase in rpm, the idle mixture is set too lean. If the rpm increases more than 50 rpm, the idle mixture is set too rich.
NOTE: Some amateur-builders, after properly setting the idle mixture/rpm to the manufacturer’s specification, increase the engine idle rpm by 100 rpm for the first 10 + hours of flight testing. This is to ensure that the engine will not quit when the throttle is pulled back too rapidly, or when power is reduced on the final approach to landing.
b. Magneto Check:
(1) The magneto checks should be smooth and the difference between both magnetos rpm drops should average about 50 rpm. The builder also should perform a ‘‘HOT MAG’’ check, to ensure against the engine, on its own, deciding when and where to start. To perform a hot mag check, run up the aircraft until the engine is warm. At idle rpm turn the magneto switch off; the engine should stop running. If the engine continues to run, one or both of the magnetos is hot (not grounded).
(2) The usual causes for a hot magneto are a broken ‘‘P’’ lead coming out of the magneto or a bad magneto switch. THIS IS AN IMMEDIATE
THREAT TO THE PERSONAL SAFETY OF ANY-
ONE NEAR THE AIRPLANE AND MUST BE REPAIRED AT ONCE.
c. Cold Cylinder Check:
(1) If the engine is running rough and the builder determines it may be an ignition problem, perform the following check:
(i) Run the engine on the bad magneto for about 30 seconds at 1200 rpm. Without switching the mag switch back to ‘‘both,’’ shut off the engine.
(ii) One of the test crew should quickly use a grease pencil to mark an area of the exhaust stacks approximately an inch from the flange that attaches the stacks to the cylinders.
(iii) Check the marks on the stacks. If one or more of the exhaust stacks with a grease mark has NOT been burned to a grayish-white color and the mark on the stack still retains most of the original color of the grease pencil, the ‘‘cold cylinder’’ has been identified.
(2) Probable causes of the cold cylinder problem are defective spark plugs, ignition leads, or a cracked distributor in one of the magnetos. To detect if the spark plugs are bad, switch both plugs to another cylinder. If the grease pencil proves the problem moved to the new cylinder, the spark plugs are bad. If the problem remains with the original cylinder, the ignition lead or magneto is bad.
d. Carburetor Heat:
(1) It is strongly recommended that all amateur-builders install a carburetor heat system that complies with the engine manufacturer’s recommendation. If no recommendation is available, the FAA suggests a carburetor heat system for a sealevel engine and a conventional venturi should be designed so that it will provide a 90 degrees F increase in the venturi at 75 percent power. For altitude engines using a conventional venturi carburetor, 120 degrees F increase in venturi temperature at 75 percent power will prevent or eliminate icing.
Remember: Too little carburetor heat will have no effect on carburetor icing, and too much carburetor heat will cause a overly rich mixture which will reduce power and may shut down the engine.
(2) During the engine tests, make numerous checks of the carburetor heat system. To avoid overly rich mixtures from oversized carburetor heat ducts, ensure that the carburetor heat duct is the same size as the inlet of the carburetor.
(3) Be certain there is a positive reduction in rpm each time ‘‘carb heat’’ is applied. If there is no reduction, or the rpm drop is less than expected, check the carb heat control in the cockpit and on the carb heat air box for full travel. Also check for air leaks in the ‘‘SCAT TUBE’’ that connects the heat muff to the carburetor air box.
e. Fuel Flow and Unusable Fuel Check: This is a field test to ensure the aircraft engine will get enough fuel to run properly, even if the aircraft is in a steep climb or stall attitude.
(1) First, place the aircraft’s nose at an angle 5 degrees above the highest anticipated climb angle. The easiest and safest way to do this with a conventional gear aircraft is to dig a hole and place the aircraft’s tail in it. For a nose gear aircraft, build a ramp to raise the nose gear to the proper angle.
(2) Make sure the aircraft is tied-down and chocked. With minimum fuel in the tanks, disconnect the fuel line to carburetor. The fuel flow with a gravity flow system should be 150 percent of the fuel consumption of the engine at full throttle. With a fuel system that is pressurized, the fuel flow should be at least 125 percent. When the fuel stops flowing, the remaining fuel is the ‘‘unusable fuel’’ quantity.
(3) Since the fuel consumption of most modern engines is approximately .55 pounds per brake horsepower per hour for a 100 horsepower engine, the test fuel flow should be 82.5 pounds (13.7 gallons) per hour for gravity feed, or 68.75 pounds
(11.5 gallons) per hour for a pressurized system. The pounds per hour divided by 60 equals 1.4 pounds and 1.15 pounds per minute fuel rate respectively.
NOTE: Formula for fuel flow rate gravity feed is .55 x engine horsepower x 1.50 = pounds of fuel per hour divided by 60 to get pounds per minute, divided by 6 to get gallons per minute. For a pressurized system, substitute 1.25 for 1.50 to determine fuel flow rate.
f. Changing Fuel Flow or Pressure: If the aircraft’s fuel flow rate is less than planned, there
24 5/24/95AC 90-89A is a volume or pressure problem. An increase in the fuel flow volume may necessitate installation of larger fuel line fittings on the fuel tanks, fuel selector, and carburetor in addition to larger internal diameter fuel lines. To increase fuel pressure, install an electrically driven or engine driven mechanical fuel pump prior to the first flight.
g. Compression Check: When the engine runin procedures have been completed, perform an additional differential compression check on the engine and record the findings. If a cylinder has less than 60/80 reading on the differential test gauges on a hot engine, that cylinder is suspect. Have someone hold the propeller at the weak cylinder’s top dead center and with compressed air still being applied, LISTEN. If air is heard coming out of the exhaust pipe, the exhaust valve is not seating properly. If air is heard coming out of the air cleaner/carb heat air box, the intake valve is bad. When the oil dip stick is removed and air rushes out, the piston rings are the problem.
h. Last Check: Drain the oil and replace the oil filter, if applicable. Check the oil and screens for metal, visually inspect the engine, and do a runup in preparation for the taxi tests. Do not fly the aircraft if anything is wrong, no matter how small or how insignificant. The sky, like the sea, is an unforgiving and uncompromising environment.
25 SECTION 12. PROPELLER INSPECTION ‘‘A tough decision is what a man makes when he cannot form a committee to share the blame’’ George Lutz, Col. U.S. Air Force, Retired (1994)
1. OBJECTIVE. To help the amateur-builder/ ultralight aircraft owner develop an inspection program to maintain his/her propeller.
a. There are three kinds of propeller designs:
metal, wood, and composite.
(1) Because of weight considerations, metal propellers are used more on amateur-built aircraft than ultralight aircraft. This makes wood and composite propellers the overwhelming choice for ultralight aircraft.
(2) Wood propellers are light, reliable, and inexpensive but require frequent inspections.
(3) Composite carbon-graphite material props are more expensive than wood, but are stronger and require less maintenance.
b. All types of propellers have one thing in common: they are constantly under high levels of vibration, torque, thrust, bending loads, and rotational stress. Even small nicks in the leading edge of the blade can very quickly lead to a crack, followed by blade separation. Propeller tip failure and a subsequent violent, out of balance situation can cause the propeller, engine, and its mounts to be pulled from the airframe in less than 5 seconds.
c. It is essential that the make and model propeller is carefully chosen. Always follow the manufacturer’s recommendations.
d. Exercise caution if experimenting with different makes and models propellers. A propeller with the wrong size and pitch will give a poor rate of climb, cruise, or could cause the engine to ‘‘overrev.’’
2. RECOMMENDATIONS FOR ALL PROPELLERS.
a. Never use a propeller for a tow bar when moving the aircraft.
b. Never stand in front of or in-line of a rotating propeller.
26 5/24/95AC 90-89A
c. Never ‘‘PROP’’ an engine on uneven or wet/snow covered ground.
d. Always inspect the propeller before and after a flight.
e. When working on a propeller, make sure the ignition is off first.
f. Always maintain the propeller to manufacturer’s instructions.
g. To avoid nicks and cuts, do not perform run-ups near gravel/loose stones.
h. Apply a coat of automotive wax once a month to protect the finish and keep out moisture.
i. Assume a propeller is unairworthy if it has suffered any kind of impact or ground strike.
j. After any repair or repainting, or if vibration or roughness is noted, re-balance the propeller.
k. Propeller blades should be balanced within 1 gram of each other to avoid over stressing the gear reduction system and propeller shaft.
l. Check the bolt torque on all newly installed propellers every hour of operation for the first 10 hours and once every 5 hours thereafter.
m. After torquing the propeller, track the blades.
FIGURE 5 - Propeller Tracking
3. PROPELLER TRACKING CHECK.
a. Ensuring good powerplant operation first starts with a properly installed propeller. Each propeller should be checked for proper tracking (blades rotating in the same plane of rotation). The following procedure is simple and takes less than 30 minutes:
(1) Chock the aircraft so it cannot be moved. Remove one sparkplug from each cylinder. This will make the propeller easier and safer to turn.
(2) Rotate the blade so it is pointing straight down.
(3) Place a solid object (e.g., a heavy wooden block that is at least a couple inches higher off the ground than the distance between the propeller tip and the ground) next to the propeller tip so it just touches.
(4) Rotate the propeller slowly to see if the next blade ‘‘tracks’’ through the same point (touches the block, see figure 2). Each blade should be within 1⁄16’’ from one another.
b. If the propeller is out of track, it may be due to one or more propeller blades that are bent, a bent propeller flange, or propeller mounting bolts that are over or under torqued. An out-of-track propeller will cause vibration and stress to the engine and airframe and may cause premature propeller failure.
4. METAL PROPELLER INSPECTION Perhaps the two biggest problems affecting the airworthiness of metal propellers are corrosion and nicks on the leading edge.
a. Identifying Corrosion.
(1) Surface corrosion can occur on the surface of metal blades due to a chemical or electrochemical action. The oxidation product usually appears on the surface of the metal as a white powder.
(2) Pitting corrosion causes small cavities or pits extending into the metal surface. This is an advanced form of corrosion, appearing as small dark holes that usually form under decals or blade overlays.
(3) Inter-granular corrosion, rare and difficult to detect in propellers, is the most dangerous form of corrosion. It attacks the boundary layers of the metal, creating patches of lifted metal and white/ gray exfoliation on the surface of the propeller. It is sometimes found in propellers that had a ground strike and have been straightened.
(4) If any of these signs of corrosion are found, do NOT fly the aircraft. Refer to the manufacturer’s maintenance manual for corrosion limits and repairs or AC 43.4, ‘‘Corrosion Control for Aircraft,’’ and AC 20-37D, ‘‘Aircraft, Metal Propeller Maintenance,’’ for additional maintenance information and corrective actions.
b. Nicks and Metal Blades.
(1) Nicks in the leading and trailing edge of a metal blade are usually V-shaped. They are caused by high speed impact between the propeller and a stone or piece of gravel. Properly trained individuals can ‘‘dress out’’ the crack if the nick is not too wide and/or deep. Before each nick is dressed out, each nick and surrounding area should be inspected with a 10-power magnifying glass for cracks. If an area looks suspicious, inspect the area again using the propeller manufacturer’s approved dye penetrant or fluorescent penetrant method.
(2) If the nick is left unattended, the high propeller operational stresses will be concentrated at the bottom of the nick’s V and, in time, will generate a crack. The crack can migrate across the blade until the blade fails, producing a massive imbalance between the propeller and the engine, ultimately causing structural failure. Cracks in metal blades CANNOT be repaired. A cracked propeller must be marked unserviceable and discarded.
c. Warning. Metal propellers are matched/ tuned to the engine and airframe resonant frequency by being manufactured with a particular diameter to minimize vibration. DO NOT SHORTEN METAL BLADES for any reason unless the manufacturer specifically permits this major alteration.
5. PROPELLER INSPECTION.
a. Wood propellers should be inspected before and immediately after a flight. Inspect to ensure the following:
(1) The drain holes are open on metal edged blade tips
(2) The metal/composite leading edge is secured and serviceable
(3) The blades, hub, and leading edge have no scars or bruises
(4) The mounting bolt torque and safety wire or cotter pins are secure
(5) There are no cracks on the propeller spinner (if applicable), and the safety wire is secure
(6) There are no small cracks in the protective coating on the propeller, which are caused by UV radiation
(7) The charring around the mating surface of the prop and the engine flange -- both indications of a loose propeller
b. A word about torque: A new, wooden propeller should have the mounting bolts checked
28 5/24/95AC 90-89A for proper torque within the first hour of flight and every hour for 10 operational hours thereafter.
(1) After 10 hours, check the bolt torque every 5 hours thereafter. The mounting bolt torque also should be checked prior to flight if the aircraft has been in storage for a long period of time (3 to 6 months).
(2) If the bolts need to be torqued, it is suggested all the bolts be loosened for an hour to allow the wood to relax. ‘‘Finger tighten’’ the bolts until snug and tighten the attaching bolts in small increments, moving diagonally across the bolt circle.
It is good practice to check the propeller track (see chapter 1, section 7) as the bolts are torqued down.
The torqued bolts should be safety wired in pairs.
(3) If nylon/fiber insert type nuts are used, they should be changed every time the propeller bolts are re-torqued. They should never be used with a bolt with a cotter key hole in the threaded area because the sharp edges around the hole will cut the nylon/fiber insert and reduce the fastener’s effectiveness. All self-locking nuts should have at least two bolt threads visible pass the nylon/fiber insert after torquing.
(4) If any of the following damage is found, a wood propeller should be removed from the aircraft and sent back to the manufacturer for repair. If the propeller cannot be saved, it should be marked unserviceable.
(i) Any cracks in the blades or hub
(ii) Deep cuts across the wood grain
(iii) Blade track that exceeds 1⁄16’’ limits after attempts to repair
(iv) Any warpage or obvious defect
(v) Extreme wear (leading edge erosion, bolt hole elongation)
(vi) Any separation between lamination NOTE: When parking the aircraft, always leave the wood propeller in the horizontal position. This position will allow the wood to absorb small amounts of moisture evenly across it’s entire span rather than concentrating the moisture (weight) in the low blade and creating a vibration problem.
6. COMPOSITE PROPELLERS INSPECTION.
a. There are generally two types of composite propellers: thermo-plastic injection molded propeller and the carbon/graphite fiber composite propeller.
(1) The thermo-plastic injection molded propeller is a low cost, thin bladed propeller used on engines of 80 horsepower or less. Propeller inspection is straight forward, by examining the blades and hub for cracks and nicks. If a crack is found, do not fly until the propeller is replaced. Small nicks of 3⁄16 of an inch or less can be dressed out and filled using a two-part epoxy.
(2) Carbon/graphite composite propellers are primarily used on engines of 40 horsepower and more. One should inspect for small hair line cracks in the gel coat. These spider cracks are usually caused by vibration generated by a mismatch of the engine and propeller combination. If a crack in the base material of the propeller other than the gel coat is found, do not fly until the manufacturer inspects the propeller.
(i) Nicks of 1⁄2 inch or less in the leading or trailing edges of carbon/graphite propellers can be dressed out and filled using a two-part epoxy. But if the nick has severed the fiberglass roving (looks like a fiberglass wire bundle) that runs hub to tip on the leading and trailing edge, do not fly. The propeller has been severely damaged and must be sent back to the factory for inspection and repair.
(ii) Before making even small repairs on a composite propeller, check with the manufacturer first. Larger nicks must go back to the factory for inspection and repair.
29 CHAPTER 2. TAXI TESTS SECTION 1. LOW SPEED TAXI TESTS ‘‘Yelling ’Clear the Prop!’ before you start an aircraft is the first of a series of well planned, choreographed steps to make you a professional.’’ Jack Crawford, Pilot, Mechanic, Airport Operator (1994)
1. OBJECTIVES. The objectives of the taxi tests are fourfold:
a. To ensure that the aircraft ‘‘tracks’’ straight and there is adequate directional control at 20 percent below the anticipated take-off speed.
b. To determine if the aircraft’s engine cooling and the brake system are adequate.
c. To predict the flight trim of the aircraft and its handling characteristics during take off and landings.
d. To allow the pilot to become proficient with the handling and braking characteristics of the aircraft.
NOTE: All taxi tests, low and high speed, should be made as if it were the first flight.
The pilot should be wearing the proper clothing, seat belt/shoulder harness and helmet and be mentally and physically prepared for the possibility of flight.
2. TAXI TESTS.
a. Prior to beginning taxi tests in a conventional (tail dragger) aircraft, the tail should be raised until the aircraft is in the approximate take-off position. The pilot should spend an hour or more in the cockpit to become accustomed to the aircraft’s takeoff position. This small but important aspect of training will help the pilot avoid overreacting to an unexpected deck angle on the first flight.
NOTE: All taxi tests should always be monitored by a minimum of one other member of the flight test team, who will watch for evidence of fire/smoke or other problems not visible to the pilot.
b. The taxi tests should begin with a taxi speed no faster than a man can walk. The pilot should spend this time getting acquainted with the aircraft’s low speed handling characteristics by practicing 90, 180, and 360 degree turns and braking action. The pilot should also remember that monitoring the oil pressure, oil temperature, cylinder head temperature, and maintaining them within limits is a critical function that must not be overlooked.
30 5/24/95AC 90-89A NOTE: The builder should be aware that some aircraft brake manufacturers have specific brake lining conditioning procedures (break-in) for metallic and non-asbestos organic linings. Proper brake lining conditioning should be completed before starting the low and high speed taxi tests.
If not properly conditioned, the brake lining will wear quickly and give poor braking action at higher speeds.
c. The pilot should check the flight instruments for operation each time the aircraft is taxied out. The compass should match the magnetic heading of the runway or taxi way the aircraft is on. When making a turn (e.g., right hand turn), the turn coordinator/turn and bank should indicate a right hand turn but the ball should skid to the left. The vertical speed indicator should read zero and the artificial horizon should indicate level.
d. After each taxi run, inspect the aircraft for oil and brake fluid leaks. No leak should be considered a minor problem. Every leak must be repaired and the system serviced prior to the next taxi test.
SECTION 2. HIGH SPEED TAXI TESTS ‘‘First get use to the fact that you are now 30 feet wide and you steer with your feet.’’ Wayne Nutsch
1. OBJECTIVE. To determine the aircraft’s high speed handling and braking parameters.
a. Propeller rotation will determine which rudder pedal is pressed to compensate for the asymmetrical thrust of the propeller blades. For example, when viewed from the cockpit, a Volkswagen automotive engine mounted in a tractor configuration will rotate the propeller counter-clockwise. In this case, the pilot must use the left rudder pedal for high speed taxi and take-off.
b. As with every part of the flight testing program, the high speed taxi tests should follow the FLIGHT TEST PLAN. Start slowly and do not progress to the next step until everyone is thoroughly satisfied with the aircraft and his/her own performance.
c. Each taxi run should be 5 mph faster than the last run until the aircraft is within 80 percent of the predicted stall speed. Prior to reaching the predicted stall speed, the pilot should test aileron effectiveness by attempting to rock the wings slightly. As taxi speeds increase, the rudder becomes more responsive and directional control will improve.
(1) In a nose gear aircraft, the pilot should be able to raise the nose of the aircraft to a take off attitude at 80 percent of the stall speed. If the nose cannot be raised at this speed, the weight and balance and CG range should be rechecked. Most likely there is a forward CG problem or the main gear is too far aft.
(2) In a tail wheel aircraft at 80 percent of stall speed, the pilot should be able to lift the tail and assume a take-off position. Again, if the tail cannot be raised, recheck the weight and balance and CG range. Most likely there is a rearward CG problem or the main gear is too far forward.
CAUTION: Heavy braking action at high speeds in tail wheel aircraft may cause directional problems (ground loops) or nose overs.
c. If runway conditions permit, duplicate each taxi test with the flaps in the take-off and landing configuration. Record the flap effects on directional control and insert the information in the draft copy of the aircraft’s flight manual.
d. Determine the approximate point on the runway where lift-off will occur and mark it with a green flag if no other existing reference is available.
e. Determine how much runway the pilot will need if it becomes necessary to abort the take-off.
This is usually accomplished by accelerating to 80 percent of lift off speed, bringing the engine back to idle, and applying heavy braking action to bring the aircraft to a full stop. After each take-off/abort test, the brakes must be allowed to COOL DOWN.
The lining must be examined carefully and replaced if necessary.
f. After determining the distance required to come to a full stop after aborting, add 30 percent to the distance. Measure that distance from the OPPOSITE end of the active runway which will be
31 used. If no existing reference is available, mark it with a red flag. The taxi tests are completed when the test pilot is satisfied with both the aircraft’s and his/her individual performance. Prior to the first flight, the aircraft should be thoroughly inspected with special attention given to the landing gear, brake system, engine, and propeller.
g. During this inspection all discrepancies must be fixed. Examine the screens/filters for metal, flush the fuel system, and clean all the screens/filters.
Perform a leak check on the engine and the fuel system by running-up the engine.
h. Notes.
(1) The first high speed taxi tests should be made in a no wind or a light head wind condition.
The pilot should ensure that the tests will not interfere with the normal airport operations or create a safety hazard for other aircraft.
(2) If the aircraft’s engine is not a U.S. type certificated engine, the pilot should determine which way the propeller rotates.
(3) Pilots of tail wheel aircraft must always be aware that ground loops are possible at any speed.
This is true especially if the main landing gear is located too far forward of the aircraft’s CG.
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33 CHAPTER 3. THE FIRST FLIGHT ‘‘It is critically important that a test pilot never succumb to the temptation to do too much too soon, for that path leads but to the grave.’’ Richard Hallion (1987) SECTION 1. GENERAL
1. OBJECTIVE. To take every precaution to ensure that the first test flight is an ‘‘uneventful’’ one.
2. GENERAL.
a. The first flight is an important event for an amateur-builder. As important as it is, it should not be turned into a social occasion. This puts enormous peer pressure on the pilot to fly an aircraft that may not be airworthy or to conduct the flight in inclement weather.
b. A ‘‘professional’’ will avoid this trap by following the FLIGHT TEST PLAN and inviting only those members of the crew needed to perform specialized tasks when testing the aircraft.
c. A safe and uneventful first flight begins with verifying all emergency equipment and personnel are standing by, radio communications are functional, members of the crew are briefed, weather is ideal, and the aircraft is airworthy. The pilot must be rested and physically and mentally ready for the first flight and every flight thereafter. The pilot also should review any new data developed for the aircraft’s flight manual.
d. The first flight should be flown a thousand times: the first 500 on paper, the next 499 flights in the test pilot’s mind -- and once in actuality. The first flight test should be so well-rehearsed by the test pilot and ground crew that the first flight is a non-event.