N632DRZenith Aircraft STOL CH 750 · 2011

On April 14, 2011, about 0940 Pacific daylight time, a Zenith CH-750 experimental amateur-built airplane, N632DR, was substantially damaged during a forced landing following a complete loss of engine power near Winchester, California. The pilot/owner, the sole person on board, received minor injuries. The personal flight was operated under the provisions of Title 14 Code of Federal Regulations Part 91, as part of the Phase 1 flight test program for the airplane. Visual meteorological conditions prevailed, and no flight plan had been filed.

According to the pilot, the accident flight was the third flight for the airplane. The pilot spent about 30 to 40 minutes conducting a pre-flight inspection, and added 4 gallons of fuel, to bring the total takeoff fuel quantity to about 13 gallons. He started the engine, warmed it up, and then conducted an uneventful engine run-up and takeoff. After takeoff, the pilot headed east towards a nearby lake, and climbed to an altitude of about 5,000 feet above mean sea level (msl). While over the lake, he conducted turns and "studied the airplane," in order to familiarize himself with the airplane's handling and operating characteristics. After about 45 minutes, the pilot decided to return to his departure airport, French Valley Airport (F70), Murrieta/Temecula, California, He turned west, reduced power to about 3/4 throttle, and began a descent. When the airplane was at an altitude of about 3,000 feet msl, the engine "suddenly died."

The pilot made three or four restart attempts; each time, the engine would catch and turn about 10 revolutions, and then stop. After a few minutes, the pilot realized that he would not reach F70, and executed a 180-degree turn to head for a straight, empty road he knew of in the area. Shortly thereafter, he noticed powerlines between his position and the road, and pushed the nose down to fly under the powerlines. After that maneuver, the airplane did not have enough altitude to reach the road, and touched down in a field, where it nosed over into the inverted position. The pilot extracted himself from the airplane. First responders observed that fuel was leaking from the tanks.

According to information provided by the Federal Aviation Administration (FAA) and the pilot, the pilot held a private pilot certificate, but no longer held a current FAA medical certificate, and was therefore operating under sport pilot regulations. The pilot reported that he had approximately 1,325 total hours of flight experience, all of which were in single engine airplanes, and which included approximately 3 hours in the accident airplane make and model. His most recent flight review was completed in March 2011.

The airplane was equipped with a modified Chevrolet Corvair engine, which was assembled by the pilot with components and guidance from an engine-modification company (William Wynne, dba "The Corvair Authority"), based in Florida. The engine was equipped with a Marvel-Schebler aviation carburetor. There were provisions for providing heated air to the carburetor ("carburetor heat") intake air. The pilot stated that the carburetor "was recently purchased as completely rebuilt from a certified supplier. The carb[uretor] is mounted at the lower rear of the engine with updraft intake manifolds to each opposed head" located approximately 18 inches above the carburetor.

The ignition system consisted of a single spark plug in each cylinder, with dual ignition coils and dual breaker points. Although required by FAA regulations, the airplane was not equipped with an emergency locator transmitter.

The airplane was equipped with two interconnected wing tanks, which were not individually selectable. The two tanks had a combined total capacity of about 24 gallons. The pilot had conducted the initial ground and taxi runs with automotive gasoline, but then switched to 100LL aviation fuel for the flight test phase. The pilot was uncertain whether the automotive gasoline contained ethanol.

Automated weather observations from three airports within a 20 mile radius of the accident location included light winds; clear skies; temperatures between 15 and 18 degrees C; and corresponding dew points between 5 and 9 degrees C.

The three sets of temperature and dew point values for the approximate time and location of the power loss indicated that the relative humidity was about 50 to 60 percent. When the intersections of the temperature values were located on a chart that depicted carburetor ice envelopes, the points were in the region of the chart denoted as "Serious Icing (Glide Power)," near the boundary of the region denoted as "Serious Icing (Cruise Power)."

The NTSB did not physically respond to the accident site. According to information provided by the FAA representative who responded to the accident, the airplane was located in a field about 2.6 miles east of F70. The airplane was inverted, the lower nose cowl and spinner were crushed, and the main landing gear had separated from the fuselage. Only trace amounts of fuel were found in the tanks, and examination of the ground under the inverted tanks also revealed only trace amounts of fuel. The FAA inspector indicated that he did not detect anything unusual in his examination of the airplane, and that the engine could be manually rotated.

At the time of the accident, the hour meter registered a total of 4.8 hours. The pilot estimated that 1 hour of that time was initial ground and taxi test time, and the remainder was flight time.

Additional Information from the Pilot

In response to questions from the NTSB, the pilot provided the following additional information regarding the airplane and accident:

- He tested the residual fuel leftover from that used to fuel the airplane for the accident flight; it tested negative for both water and ethanol.

- He checked the fuel strainers in each wing fuel tank and in the gascolator "for residue and found very little of anything that could have plugged up the flow of fuel."

Regarding the use of carburetor heat on the accident flight, the pilot provided the following written response:

"Carb heat was not applied because I never did throttle back to a low rpm for landing. I had been flying for approx. 45 minutes at approx. 2,750 rpm with no problems and had slowed down to about 2,400 rpm in order to descend to pattern altitude. I don't normally apply carb heat until I slow the engine down below 2,000 rpm. I was at 2,400 rpm (without carb heat) for about 30 seconds when the engine stopped suddenly. 4 attempted restarts resulted in a slight sputtering at very slow rpm without starting, and I interpreted that as a fuel starvation problem."

Information Provided by the Engine Modification Designer

The following text was excerpted directly from an email from the chief designer at the engine modification facility regarding the engine:

- The brake specific fuel consumption is .45 lbs/hp/hr in cruise, and .5 lbs/hp/hr at full power.

- The 120HP engine would consume about 6.75 gallons per hour at 75% (cruise) power

- The 120HP engine would consume about 10 gallons per hour at full (climb) power

- These are base line numbers. The plane had a ground adjustable prop on it, and if the pitch is set slightly flat, it will allow the engine to produce in excess of 120 hp. The fuel consumption could actually be higher than the above numbers, but not lower.

- The actual rate of climb of the accident airplane is relatively low, and "he may have spent a lot longer in climb" than the pilot estimated, which could result in an overall fuel consumption higher than expected.

Engine Examination

About 1 month after the accident, the engine designer examined the wreckage and produced a report, which he posted on his website (www.flycorvair.com). That examination was conducted without prior knowledge of, and independent of, any NTSB or FAA involvement, and the results were not verified by any NTSB or FAA personnel. The following is a brief synopsis of the examination results:

- The airframe nosebowl is utilized on the Zenith 601 and 750 airplanes. Typically, the air intake openings on the nosebowl are trimmed to 4.25 inch diameter for a Zenith 601 or 650, and 4.5 inch diameter for a 750. The holes on the accident airplane measured 3.75 inches in diameter.

- The fuel system design caused the owner/pilot to have difficulties "calibrating the quantities in each individual tank and making his electronic instrumentation read this correctly. This led to him flying with inoperative fuel gauges." Operation with inoperative gauges would not allow the pilot "to notice any fuel leakage in flight, nor unexpectedly high fuel consumption from operating a very powerful engine on a high drag airframe" such as the accident airplane.

Engine Test Run

As part of his examination of the airplane/engine, the designer conducted a test run of the engine. The preparations and results are summarized below

- The wings, which contained the fuel tanks, had been removed for airplane recovery

- A new replacement fixed-pitch propeller was installed, since the accident propeller was damaged

- A 1-gallon can for fuel was secured to the roof of the airplane

- All engine controls, including the throttle, were used as-is

- "I literally did not touch anything else on the engine, make any adjustment of any kind, charge the battery, or touch a wrench or screwdriver on anything. I simply turned the ignition switch on, primed the engine by moving the carburetor throttle twice, pressed the starter button and the engine ran after 2 seconds of cranking.…We did two engine runs about 5 minutes long each at various power settings. The engine not only made full power but also idled, and restarted normally. It ran equally on both ignitions."

- The designer concluded that the engine failure was due to fuel exhaustion