12 5/24/95AC 90-89A
4. FIELD CHECK. Two people are needed to accomplish the following field check that will enable an amateur-builder to detect if the aircraft’s instrument system is leaking: (Note: This field check is not an accuracy check.)
a. Airspeed check: Slip a long rubber hose over the pitot mast (surgical tubing is recommended).
As one person reads the airspeed, the other should very slowly roll up the other end of the tubing. This will apply pressure to the instrument. When the airspeed indicator needle reaches the aircraft’s approximate recommended cruise speed, pinch the hose shut, and hold that reading. The airspeed needle should remain steady for a minute if the system is sound.
A fast drop off will indicate a leak in the instrument, fittings, lines, or the test hose attachment. NEVER force air in the pitot tube or orally apply suction on a static vent. This will cause damage to the instruments.
b. Altimeter/vertical speed check.
(1) To check the static side, apply low suction at the end of the static vent port. The easiest way to gain access to the static system is to remove the static line at the static port. If there are two static ports, tape the unused port closed. Next, get two feet of surgical tubing, seal one end, and tightly roll it up. Attach the open end to the static line and slowly unroll the tubing. This will apply a suction, or low pressure, to the static system.
(2) The altimeter should start to show an increase in altitude. The vertical speed indicator also should indicate a rate of climb. The airspeed may show a small positive indication. When the altimeter reads approximately 2,000 feet, stop and pinch off the tube. There will be some initial decrease in altitude and the vertical speed will read zero. The altimeter should then hold the indicated altitude for at least a minute. If altitude is lost, check for leaks.
(3) IMPORTANT: The above airspeed and altimeter field checks should not be considered the equivalent of airspeed or static system accuracy tests as certified by a certificated repair station, but a check of the system for possible leaks. These checks do not take into consideration the pitot tube and static ports located on the airframe. The FAA recommends the builder not deviate from the designer’s original plans when installing the pitot and static system.
c. Fuel system: Since 1983, more than 70 percent of the engine failures in amateur-built aircraft were caused by fuel system problems. Many times the direct cause of engine failure was dirt and debris in the fuel tank and lines left behind during the manufacturing process.
(1) Before the aircraft’s fuel tanks are filled, the amateur-builder should vacuum any manufacturing debris from each tank and wipe them down with a ‘‘tack’’ cloth (available from a paint supply store). Next, the system should be flushed with aviation grade gasoline several times in order to remove any small or hard to reach debris from the tanks and lines. The fuel filter/gasolator screen/carburetor finger screen should also be cleaned. The amount of time spent ‘‘sanitizing’’ the fuel system will provide big safety dividends for the life of the aircraft.
(2) When filling the tanks, place the aircraft in the straight and level cruise position. Add fuel in measured amounts to calibrate the fuel tank indicators. While allowing the aircraft to sit for a short time to observe for possible leaks, inspect the fuel tank vents to see if they are open and clear.
Check that the fuel tank caps seal properly. If there are no leaks and the fuel system has an electric boost pump, pressurize the system and again check for leaks. The fuel selector, vents and fuel drains should be properly marked and tested for proper operation.
NOTE: Many amateur-built aircraft take 5 to 8 years to build. During that time, many rubber-based oil and fuel lines and cork gaskets that were installed early in the building process may have age hardened, cracked, and/or turned brittle. The builder should carefully inspect these components and replace as necessary to prevent a premature engine failure.
d. Hydraulic system: The hydraulic system should function dependably and positively in accordance with the designer’s intent. Retractable landing gear should be rigorously cycled on the ground, using both the normal and emergency landing gear extension system.
e. Safety belt and shoulder harness: These items should be checked for condition and proper installation. A review of amateur-built aircraft accidents has disclosed a significant number of accidents in which the seat belt mounting hard points
13 failed. Each seat belt and shoulder harness mounting hard point should be built to the designer’s specifications to ensure that it will hold the harness and pilot in the aircraft at the ultimate design ‘‘G’’ load specification, both positive and negative, for the aircraft.
f. Avionics and electrical checks: Test the avionics systems. Perform an operational check to ensure the radio(s) transmit and receive on all frequencies. Inspect circuit breakers/fuses, microphones, and antennas for security and operation. Test the ELT for proper operation and battery life. Electrical systems can be checked for operation of lights, instruments, and basic nav/comm performance.
Other electrical systems, such as generator/alternator output can be checked during the engine run-ins, taxi, and flight tests. Check the battery and the battery compartment for security and if applicable, ensure that the battery is properly vented to the outside of the aircraft. Check the condition of the engine to airframe bonding (grounding) wire. Ensure that all electrical instruments operate properly.
g. Cowling and panel checks: Ensure that all inspection panels are in place, the cowling is secured, and cowl flap operation is satisfactory. Inspect the propeller spinner and its backing plate for cracks.
h. Canopy/door locks checks: Ensure the canopy or doors on the aircraft work as advertised. Double check the canopy or door lock(s) so the canopy and doors will not open in flight and disturb the airflow over the wings and stall the aircraft. If a canopy jettison system is installed, check for proper operation when the aircraft on the ground and when it is on jacks. (Jacks will simulate flight loads on the aircraft.)
14 5/24/95AC 90-89A SECTION 8. WEIGHT AND BALANCE ‘‘Never argue with your spouse or a mathematician’’ Phil Larsh, Accident Prevention Counselor, Colfax IN (1994)
1. OBJECTIVE. To discuss the importance of developing an accurate weight and balance calculations for both test and recreational flights. Additional information on weight and balance can be found in AC 91-23A, Pilot’s Weight and Balance Handbook.
a. A good weight and balance calculation is the keystone of flight testing. Accurately determining the aircraft’s take-off weight and ensuring that the center of gravity (CG) is within the aircraft’s design for each flight is critical to conducting a safe flight test.
b. An aircraft should be level when weighed, spanwise and fore and aft in accordance with the manufacturer’s instructions, and should be in the level flight position. It is highly recommended that the weighing be done in an enclosed area, using three calibrated scales. Bathroom scales are not recommended because they are not always accurate.
ITEMS WEIGHT (LBS) ARM (INCHES) MOMENT (IN-LBS) Left Wheel Right Wheel Tail Wheel TOTALS 101 99 42 242 60 60 180
80.8 6060 5940 7560 19560 TOTAL MOMENT = Empty weight CG or 19560 = 80.8 TOTAL WEIGHT 242 FIGURE 2. EMPTY WEIGHT CG
2. DETERMINING EMPTY WEIGHT CG.
a. The sample airplane for determining empty weight is a single seater, which the kit manufacturer’s design empty weight of 253 pounds and a gross weight limit of 500 pounds. The datum line is located at the nose of the aircraft and the CG range is between 69 to 74 inches from the datum.
b. To work a CG problem, figure the EMPTY WEIGHT CG first. On a piece of paper draw four blocks. Title each block from left to right as shown in figure 3.
(1) Under the block titled item, vertically list ‘‘left wheel,’’ ‘‘right wheel,’’ and ‘‘nose/tail wheel.’’
(2) Place a calibrated scale under each wheel and record the weight on each gear, in pounds, in the weight block along side the appropriate wheel.
This process is done with an empty fuel tank.
(3) Measure in inches the distance from the datum line, or imaginary point identified by the manufacturer (e.g., nose of the aircraft), to the center line (C/L) of the three wheels. Record the distance of each wheel and place it in the moment arm block beside the appropriate wheel (see figure 2).
(4) Multiply the number of inches (arm) by the weight on each wheel to get the moment (inchpounds) for each wheel. Add the weight on the three gears and the three moments in inch pounds and divide the total weight into the total moment. The sum is the ‘‘EMPTY WEIGHT CENTER OF GRAVITY’’ in inches. In the sample case, the empty weight CG is 80.8.
NOTE: All calculations should be carried out to two decimal places.
16 5/24/95AC 90-89A
ITEMS WEIGHT (LBS) ARM (INCHES) MOMENT (IN-LBS) A/C Pilot Fuel TOTALS 242 170 30 442
80.8 65 70 74 19560 11050 2100 32710 TOTAL MOMENT = Takeoff CG or 32710 = 74 TOTAL WEIGHT 442 FIGURE 3. TAKE-OFF CG
3. DETERMINING TAKE-OFF WEIGHT CG.
a. Since the aircraft’s empty weight and empty weight CG are fixed numbers, the only way an aircraft’s CG can be changed is by adding weight in other locations.
b. For example, in figure 3, the aircraft’s empty weight has been written in the appropriate blocks. The pilot weighs 170 pounds and fuel (5 gallons) weighs 30 pounds.
c. Again, all measurements are made from the datum to the center line of the object that has been added. Weight multiplied by inches from the datum equals moment. Add the weights and moments to find the take-off CG for that particular flight.
d. Loaded in this configuration, the aircraft is within the CG flight envelope and is safe to fly.
ITEMS WEIGHT (LBS) ARM (INCHES) MOMENT (IN-LBS) A/C S/B Pilot Strobe Fuel Fuel TOTALS 242 15 170
1.5 30
1.5 460
80.8 75 65 179 70 55
74.3 19560 1125 11050
268.5 2100
82.5 34186 TOTAL MOMENT = Alteration Takeoff Weight CG or 34186 = 74.3 TOTAL WEIGHT 460 FIGURE 4. ADDITIONAL EQUIPMENT ADDED
4. ADDING ADDITIONAL EQUIPMENT.
a. During flight testing, a strobe battery and hand held radio are added. The battery/battery box weight is 15 pounds and the location is 75 inches aft of the datum; the strobe assembly weight is 1.5 pounds and is located 179 inches aft of the datum;
the radio’s weight is 1.5 pounds and is located 55 inches aft of the datum (see figure 4).
b. In the sample problem, the previous figures for take-off weight and moment are still accurate, hence those numbers have been listed in the appropriate blocks.
(1) Add the battery, strobe, and radio numbers in the appropriate locations and calculate the totals. At 465 pounds, the aircraft is still 35 pounds under its design gross weight limit of 500 pounds but is out of balance because the CG has moved .3 inches further aft (74.3 inches) than the allowable rear CG limit of 74 inches.
(2) Since the aircraft is out of balance with an aft CG, it is no longer 100 percent stable in pitch and would be dangerous to fly. In most cases, it is not the amount of weight added to the aircraft that can cause a major safety problem but its location.
(3) To bring this aircraft back into the safe CG range, the battery would have to be moved 9 inches forward (66 inches from the datum line).
Another alternative is to install 8 pounds of ballast in the nose (20 inches from the datum).
(4) If the sample aircraft exceeded the designer’s gross weight limit (e.g., 300 pound pilot) instead of the CG limit, its climb, stall, and performance capability would be poor and the possibility for in-flight structural failure would be high.
NOTE: In the sample weight and balance, positive numbers were chosen by placing the datum line on the nose of the aircraft. Some manufacturers prefer to use a datum located somewhere between the aircraft’s nose and the leading edge of the wing.
(5) This kind of datum will set up a system of positive arms (items located aft of the datum) and negative arms (items located forward of the datum).
(6) When working a weight and balance problem with negative and positive moments, subtract the sum of all negative moments from the sum of all positive moments to reach a ‘‘total moment’’ for the aircraft.
18 5/24/95AC 90-89A SECTION 9. PAPERWORK ‘‘It is harder to write a lie in a logbook than tell one, because your eyes see it and your fingers feel it.’’ Bob Moorman, Ultralight Instructor (1994)
1. OBJECTIVE. To have the proper documentation and paperwork to conduct the flight test.
a. Weight and Balance: The weight and balance for the aircraft should be carefully done. The gross weight and CG range should be determined prior to every flight.
b. Airworthiness/Registration/Operating Limitations/Placards/Weight and Balance: Must be on board, or the aircraft is not legal to be operated.
c. Checklists: In addition to the assembly/airworthiness checklist previously discussed in section 7, the builder should prepare the following checklists: preflight; take-off/cruise; before starting;
descent/before landing; starting the engine; after landing; before takeoff; securing the aircraft; and emergency procedures. A checklist to cover the above procedures may seem a tedious task, but it will only be the size of a 5x8 card -- similar to a checklist for a Cessna 150 or a Piper PA-28-140.
NOTE: The amateur-builder should anticipate several revisions to the checklists.
d. Flight Manual: It is imperative a flight manual describing the anticipated performance of the aircraft be written by the aircraft builder/kit manufacturer. The manual will be revised several times during the flight test phase until it accurately reports the aircraft’s performance.
e. Maintenance Records (logbooks): Operators of amateur-built aircraft are required to only record the yearly condition inspections in accordance with the aircraft’s operating limitations. The FAA recommends, however, that every amateur-built aircraft/ultralight owner record in the aircraft’s logbooks all inspections and maintenance performed.
This will create an aircraft’s maintenance history and will be invaluable in spotting trends.
19 SECTION 10. POWERPLANT TESTS ‘‘Don’t short-change the engine tests or you won’t be around to give your grandkids a ride.’’ Dick Koehler, A&P Instructor (1994)
1. OBJECTIVE. To ensure that the engine has been properly run-in and is safe to operate in all rpm ranges.
a. An engine pre-oil and cold compression test can be conducted as follows:
(1) Remove the rocker-box covers and one spark plug from each cylinder.
(2) Using an external oil pump, or by rotating the propeller in the direction of rotation, pump a substantial supply of oil up from the sump into the rocker arms.
(3) When the engine is pre-oiled, run a cold compression test of each cylinder.
(4) The results will serve only as an initial bench mark for comparing other compression tests taken after the engine has been run-up to operating temperature.
b. New/newly overhauled engine run-in procedures:
(1) Most amateur-builders start with a new or newly overhauled engine and proceed to ‘‘run it in’’ on the airframe. This practice is followed due to lack of access to a test cell or a special ‘‘club’’ propeller that is specifically designed to aid in engine cooling during run-in. There are pros and cons to using an airframe to run in an engine, but the best advice has always been to follow the engine manufacturer’s instructions. These instructions are found either in the manufacturer’s overhaul manuals, service bulletins, or service letters. Following the manufacturer’s instructions is especially important if the engine has chrome cylinders which require special run-in procedures.
(2) Also, before running-up the engine, be certain that it has the proper grade oil in the sump.
Some new and newly overhauled engines are shipped with a special preservative oil to prevent corrosion.
20 5/24/95AC 90-89A Drain this out and reservice the engine with the correct oil before starting.
c. Used engine run-in procedures: Some amateur-builders install a used engine from a flyable aircraft. The same checks and adjustments used on a new or newly overhauled engine should be conducted. New and used engines require special attention to engine cylinder baffling to ensure cylinder cooling is within the engine manufacturer’s cylinder head temperature specifications.
d. Pre run-in checks:
(1) Before beginning the powerplant tests, inspect the engine and propeller carefully. All fuel and oil line connections should be tight. Check the torque on the engine mount attaching bolts. Be certain that there are no tools, hardware, or rags laying between the cylinders or under the magnetos.
(2) Check for the proper amount of oil in the engine and that the dip stick gives an accurate reading of the oil quantity. Be advised that some engines were mounted on an angle in type certificated aircraft. These engines have a special part number oil dip stick, which corrects for the different angle of oil in the crankcase. The same engine, mounted level in a amateur-built aircraft with the original dip stick, will not show the correct oil quantity.
e. Test and Support Equipment:
(1) A cylinder head temperature gauge (CHT) is needed to ensure that all cylinders are receiving the proper flow of cooling air.
(2) On the newer aircraft engines, the cylinders are drilled and tapped to accept a bayonet type of CHT thermocouple probes. For older engines, the thermocouple is designed like a spark plug washer and fits under a spark plug. It can be installed in any cylinder, either under the top or bottom spark plug.
(3) Each type of CHT design can have multiple thermocouples which are connected to a selector switch in the cockpit. The pilot then selects the cylinder he wants to monitor. This also is an excellent troubleshooting tool for identifying fouled plugs and bad ignition leads.
(4) If there is only one CHT thermocouple, attach it to the rearmost cylinder on the right side of the engine (as viewed from the cockpit) and runup the engine. Run the same test on the opposite rearmost cylinder to be certain the hottest running cylinder was selected. Calibrated oil pressure and oil temperature gauges also are needed to test the accuracy of the engine instruments installed in the aircraft.
(5) The following support equipment is needed: 50 feet or more of tie-down rope, tie-down stakes, two chocks for each wheel, fire extinguisher, assorted hand tools, safety-wire, cotter-pins, ear and eye protection, grease pencils, logbooks, clip board, pen and paper, a watch to time the tests, rags, and manufacturer’s instructions.
f. Safety Precautions: Before the first engine run, ensure the aircraft is tied down, brakes on, and the wheels are chocked. The builder and flight test team should wear ear and eye protection. All flight test participants should be checked out on fire extinguisher use and operation. During engine runs, do not allow anyone to stand beside the engine, or inline or close to the propeller. Making minor adjustments to a running engine, such as idle and mixture settings, is a very dangerous procedure and should be done with great care by experienced individuals.
g. The First Engine Run:
(1) The first start of the engine is always a critical operation. The engine should be pre-oiled in accordance with the manufacturer’s instructions.
For aircraft using other than FAA-approved oil pressure and temperature gauges, the FAA recommends attaching an external calibrated oil temperature and pressure gauge to the 4 cycle engine in order to calibrate the engine instruments. After priming the engine and completing the starting engine checklist items, the first concern is to get an oil pressure reading within the first 20 to 30 seconds. If there is no oil pressure reading -- shut down.
(2) There are three common problems that would cause low or fluctuating oil pressure.
(i) Air in the oil pressure gauge line:
This is easily fixed by loosening the line connection near the oil pressure gauge and squirting oil into the line until full. Another option is to use a preoiler to provide the pressure and carefully bleed the air out of the line near the oil gauge by loosening the B-nut that connects the oil line to the gauge.
(ii) A misadjusted oil pressure relief valve: Cleaning the pressure relief ball, checking for the proper number of washers, correcting spring tension, and re-adjusting the setting could solve the problem.
(iii) An internal problem within the engine (most likely the oil pump): An engine tear down would be required.
(3) With good oil pressure/temperature readings and the engine running smoothly, ensure that the engine oil pressure and temperature gauges in the cockpit match the calibrated oil pressure and temperature gauges, which were attached to the aircraft for the first run. Do not overlook this test. It is critical to determine the accuracy of the cockpit engine gauges not only for the ground engine runin period, but for in-flight engine cooling tests.
(4) Work through the engine manufacturer’s run-in schedule. The majority of the engine manufacturers recommend a series of engine runs from low rpm to maximum rpm. Each run usually incorporates a 200 rpm increase and lasts no longer than 10 minutes. The secret to a successful engine run is not to let the engine temperatures exceed manufacture’s limits during engine runs.
NOTE: Engines with chrome cylinders or chrome rings require different high power run-in programs. Follow the manufacturer’s run-in instructions to ensure the engine will perform satisfactorily over its lifetime.
h. Engine Cool Down: After a ground-run, the cooling off period takes approximately an hour. This is because a newly overhauled engine needs time for the internal parts (e.g., rings, cylinders, valves, bearings, and gear faces) to expand and contract several times to obtain a smooth surface that retains its ‘‘memory.’’ This is a lengthy process even when done right, but it is important not to skip any of the recommended runs to save time. To do so is to risk increasing oil consumption and reducing overall engine performance, reliability, and engine life span -- which could be costly in the long-term.
i. Record the engine run-in data: During the engine run, monitor the cylinder head temperatures, oil temperature, and oil pressure. Record the readings and adjustments for future reference. If the cylinder head temperatures are rising close to the red line, reduce power and stop the test. Some causes of high cylinder head temperatures include using spark plugs with the improper heat range; cylinder head temperature gauges installed on the wrong cylinder; missing or badly designed cylinder head cooling baffles; partially plugged fuel nozzles (applicable to fuel injected engines); fuel lines of improper internal diameter (creates lean mixtures); engine improperly timed either mechanically and/or electrically; and the carburetor fuel mixture set excessively lean.
j. After shut-down:
(1) After each engine run, check for fuel and oil leaks, loose connections, and hot spots on cylinders (burnt paint). The FAA recommends draining the oil and removing the oil screen/filter within the first 2 hours of running the engine. Check the screen/filter for ferrous metal with a magnet. Wash and inspect the screen/filter for non-ferrous metal like brass, bronze, or aluminum.
(2) A very small quantity of metal in the screen is not uncommon in a new or newly overhauled engine. It is part of the painful process of ‘‘running-in.’’ If subsequent oil screen checks (2 hours apart) show the engine is ‘‘making metal,’’ this indicates a problem inside the engine and a tear down inspection is required.
(3) It also is recommended all fuel sumps, filters, and gasolators be checked for debris after each engine run. Special attention should be given to the fuel system by the builder who constructed fuel tanks out of composite or fiberglass materials.
Composite and fiberglass strands can be very fine, making visual detection difficult. Frequent cleaning of the fuel filters and screens early in the flight testing phase will avoid a gradual build up of loose composite fibers, which would reduce or stop the flow of fuel to the engine.
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