LONGMONT, CO, USA
N430N
Cessna 320A
Shortly after lift-off, the right engine lost power, and the pilot made a wheels-up landing in an alfalfa field. Examination of the right engine's top spark plugs showed evidence of operating at a rich mixture. Both auxiliary fuel pumps were on during the takeoff roll. Cessna Service Bulletin MB88-3, requiring the removal of the fuel pressure sensing system and auxiliary fuel pump switches, had not been accomplished. Functional testing later proved the auxiliary fuel pump was operating but an excessively rich fuel mixture was being delivered to the right engine by the fuel servo.
On October 18, 1996, approximately 1230 mountain daylight time, a Cessna 320A, N430N, was substantially damaged when it collided with terrain shortly after taking off from Longmont, Colorado. The airline transport pilot was not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the personal flight being conducted under Title 14 CFR Part 91. The flight was originating at the time of the accident. The pilot said both engine auxiliary fuel pumps were in the ON/ARMED (up) position as he took off on runway 29. As the airplane passed 50 feet and he retracted the landing gear, the right engine lost power. The pilot said he attempted to fly the airplane "for maybe two seconds" but it would not climb, and he made a wheels up landing in an alfalfa field. Asked why the airplane would not fly on one engine at its estimated takeoff weight of 4,800 pounds (MGTOW is 5,200 pounds), the pilot submitted the following statement: "From all the training I have given in this make and model, I have found that the Cessna 320 will demonstrate a 400 feet per minute rate of descent and the windmilling propeller will do another 500 feet per minute rate, for a total of 900 feet per minute during the "drag demo(nstration)" according to the private and commercial practical test standards. No one knows just what the drag is on the gear in transition, when the gear doors open and the whole underneath side of the wing is open. The Pilot's Operating Handbook (see attached sheets) claim that a new Cessna 320 at 4,800 pounds (approximate weight at takeoff) is just 335 feet per minute with gear and flaps up, feathered prop and 30 degrees (F.) above standard (temperature). Therefore, 900 feet per minute descent due to gear in transit and windmilling prop minus 335 feet per minute proposed climb rate equals 565 feet per minute descent, or approximately 10 feet per second. With the normal gear retraction speed of 10 to 12 seconds and the prop feathering time (human plus mechanical delays) of 7 to 10 seconds, the airplane is going to descend at that point in time. To give you a better sense of the timing, I put the gear switch in the up position, and assuming a 10 second retraction time, the gear was able to get into the wells but the inner gear doors were ripped off after I hit the grass. Two seconds to react and know the airplane wouldn't fly, 6 seconds to descend from 50 feet and land under control, with 2 seconds left to crunch the gear doors. I did the absolute best I could..." On November 7, 1996, the airplane was examined by an FAA airworthiness inspector and a representative of Teledyne Continental Motors. The landing gear was up (the gear doors were still open) and the flaps were retracted. Continuity was established between cockpit controls and the engines and propellers. The right fuel control unit (FCU) contained fuel and the screen was clean. Although the safety wire was broken, the screen was tight. Both engine fuel selectors were positioned on the main tanks. The right engine's top spark plugs were covered with a black, carbonaceous, sooty material, consistent with engine operation at a rich mixture. The left engine's top spark plugs were normal. According to the pilot (and verified by maintenance records), Cessna Service Bulletin MEB88-3 had not been accomplished. MEB88-3 requires the removal of the automatic fuel pressure sensing switch and auxiliary fuel pump switches, and replaced by direct pilot-activated, three-position lever, lock-type toggle switches. According to the Service Bulletin, the automatic fuel pressure sensing switch was originally designed to detect a loss of fuel pressure (such as would occur if the engine-driven fuel pump were to fail, which would in turn cause a loss of engine power). If properly maintained and armed by the pilot, the switch is supposed to automatically activate the auxiliary fuel pump to the HIGH mode for continued engine operation. If the pilot fails to arm the system, or if the system malfunctions (not always detectable), standby power-sustaining fuel pressure would not be available at a critical time of need. In addition, if the system were to malfunction and cause an uncommanded activation of the auxiliary fuel pump to the HIGH output mode, the engine would receive excessive fuel, flood, and lose power. It was decided to functionally test the engine on February 17, 1997. Present were the pilot and representatives from the Cessna Aircraft Company and Teledyne Continental Motors. The airplane was lifted by crane and secured to the back of a trailer. The right propeller was changed and 5 gallons of fuel was added to each main (tip) tank. Prior to starting the right engine, both auxiliary pumps were individually activated. The left pump sounded normal. The right pump emitted a "clattering" sound. With both mixture controls in idle cutoff, the left and right fuel flow gauges indicated 4 gph and 0 gph (gallons per hour), respectively. With both mixture controls at full rich, the right fuel flow gauge indicated 4 gph. The fuel supply line was removed from the fuel manifold and the auxiliary fuel pump was switched on. Fuel was discharged from the line. The right fuel selector was switched to crossfeed (left tank). There was no change in fuel flow. Initial attempts to start the right engine were not successful because, with the auxiliary fuel pump on, no fuel flow was indicated. The engine was cross-fed to the left fuel tank and the left auxiliary fuel pump was activated to prime the engine. The engine then started. When the mixture was advanced to full rich with the auxiliary fuel pump in the LOW position, the engine emitted dark black smoke and lost power. When the auxiliary fuel pump was turned off, the engine ran smoothly. It was the opinion of the representatives from the Cessna Aircraft Company and Teledyne Continental Motors that the automatic fuel pressure sensing switch had not malfunctioned, but that the fuel servo was causing an excessively rich mixture that inhibited power from being developed.
The partial failure of the fuel control servo, causing an excessive flow/output of fuel to, and flooding of, the right engine.
Source: NTSB Aviation Accident Database
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