April, 1998

DETONATION-INDUCED DAMAGE
By Bill Rothe (A&P, IA, DME, CFII,ASC-AW)

I would like to discuss a particular phenomena that can induce severe engine damage while performing what would appear to be a normal engine operation. Detonation is an uncontrolled explosion of the unburned air/fuel mixture in the engine combustion chamber. This burning could be compared to the burning of a firecracker while a normal combustion flame front might more closely resemble that of extremely fast burning candle being burned at both ends (dual ignition). This slower release of energy associated with normal combustion allows the pressure (which pushes on the pistons and provides the force to turn the propeller) to sustain its push for a greater length of time. This provides for a much more efficient transfer of energy from the fuel to the engine. Another more serious problem associated with detonation is the excessively high pressures and heat that is developed at the instant this spontaneous combustion occurs. These excessive pressures and heat can often exceed the design limits of many primary engine components (cylinders, pistons, valves and connecting rods) and result in catastrophic engine failure.

Detonation occurs primarily because the fuel has exceeded its maximum operating pressure (known as critical pressure) or its maximum operating temperature (known as critical temperature). These critical pressures and temperatures are closely linked to the octane rating of the fuel. Higher octane rated fuel have higher critical pressures and temperatures. Anything that an operator does to an engine which might cause it to exceed its maximum manifold pressure or temperature limitation will most likely cause detonation. Operating at these maximum limitations with higher octane fuels might reduce the likelihood of detonation, but may bring with it additional problems. Different octane fuels burn at different rates, with the higher octane fuels burning slower. Without changes to the ignition timing (which some engine manufacturers have approved for some engines) the end result is a slight reduction in engine performance. Additional problems could range from increased spark plug fowling to part failure resulting from sustaining the increased operating pressures and temperatures.

Detonation develops in varying degrees, depending on factors like the amount of unburned air/fuel mixture in the cylinder and the position of the piston at the instant detonation occurs. The more of this unburned air/fuel mixture there is, the more severe the pressures and temperatures rise. Unless the detonation is heavy, there is no cockpit evidence of its presence. However, an engine that has been experiencing detonation will show evidence at teardown. It should be noted that although the instantaneous pressures in the cylinder at the moment of detonation is much greater than during normal combustion, the force transmitted is much less. This reduced transmission of force is reflected as a loss of engine power.

There are a variety of causes for detonation, most of which can be prevented through proper operation and maintenance of the engine. Engines are most vulnerable to detonation at high power output when pressures and temperature are their highest. Some engine designs are more susceptible to detonation problems than others. A good example of this would be turbocharged engines. Improper operation of the turbocharger or a malfunctioning controllers could increase the possibility of overboosting the manifold pressure. This could cause normal combustion pressures to exceed the fuels critical pressure, resulting in detonation. High manifold pressure along with low R.P.M. would allow the cylinder too much time to fill with its air/fuel charge, resulting in a higher volume charge and creating higher pressures (exceeding critical pressure). There are many operational techniques that could result in exceeding critical temperatures. Some examples would be improper cowl flap and/or carb heat usage, prolonged ground operation, improper leaning technique, low oil level operation, extended high power slow flights (all causing high C. H. T.). Among the maintenance related problems that could contribute to a detonation problem are improper ignition timing (advanced), plugged injector nozzles, intake leaks, obstruction to the cooling air flow, worn rings (dilutes combustion fuel with oil in the combustion chamber), contaminated fuel, wrong heat range spark plugs, just to name a few.

Remember, the Pilot's Operating Handbook is the final authority regarding the operation of your engine. So study it thoroughly.

SAFETY SEMINARS

Pilot and Mechanic Safety Wings/AMT Meeting - Kauai
Date: Tues., April 7, 7-8:30 P.M.
Where: Civil Air Patrol - Commuter Terminal

Pilot and Mechanic Safety Wings/AMT Meeting - Hilo
Date: Tues., April 14, 6:30-8 P.M.
Where: Interisland Terminal - State Conference Room, 2nd floor

Pilot and Mechanic Safety Wings/AMT Meeting - Oahu
Date: Thurs., April 23, 7-9 P.M.
Where: Honolulu Flight Standards District Office - upstairs conference room

Pilot and Mechanic Safety Wings/AMT Meeting - Maui
Date: Tues., April 28, 7-8:30 P.M.
Where: Cameron Center Auditorium

For more information about any of these Pilot-Mechanic Safety meetings, contact Jim Hein (808) 837-8335.

OTHER ANNOUNCEMENTS

Hawaiian Flyers Toastmasters invites anyone interested in improving their listening and speaking skills to attend our fun-filled meetings. We meet on the first and third Thursdays of each month from 5:30 to 6:30 P.M. at the Honolulu FSDO office. For more information, call Dennis Noll (808) 837-8324 or Jim Hein (808) 837-8335.

HOW TO OBTAIN A COPY OF YOUR ACCIDENT/INCIDENT/ENFORCEMENT RECORDS

For a copy of your accident/incident/enforcement records, mail or fax a signed letter stating your full name, date of birth, social security number and/or certificate number(s), and your current mailing address to:

Federal Aviation Administration
ATTN: AFS-620
P.O. Box 25082
Oklahoma City, OK 73125
Fax number (405)954-4655

There is no charge to obtain a copy of these records. You should receive your copy in two weeks. If you have any questions, you may call AFS-620 at (405)954-4173.

NOTE: If you are requesting accident/incident/enforcement records on an individual other than yourself, you will need to follow the procedure listed in Advisory Circular 120-68, Pilot Records Improvement Act of 1996.

HOW TO OBTAIN A COPY OF YOUR AIRMAN CERTIFICATION RECORDS (FULL RECORD)

For a copy of your "full record" of all of your airman certification records (medical records, knowledge test results, accident/incident/enforcement history, airman certification applications, etc.), mail a signed letter stating your full name, date of birth, social security number and/or certificate number(s), and your current mailing address to:

Federal Aviation Administration
Airman Certification Branch, AFS-760
P.O. Box 25082
Oklahoma City, OK 73125

Include a check or money order for 10 dollars (U.S.), made payable to the U.S. Treasury. If the cost for copying your airman certification records is less than 10 dollars, a refund will be sent to you.

If you have any questions, you may call AFS-760 at (405)954-3261.

WINGS PROFICIENCY AWARD PROGRAM

PHASE I
Laureli Lunn

PHASE III
Daniel Bowen
Raymond Lahm

PHASE V
Frank Baker

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