No airplane is perfect for every mission. Designers make compromises in the hope of producing an airplane that will find enough buyers to be a marketing success. But buyers have proven they sometimes want what manufacturers won’t provide, so we have a lively market in aftermarket mods. One popular mod is for more powerful engines in airframes like the Cessna 172. The results include better climb, a little faster cruise and, unfortunately, higher fuel burn.
All aftermarket modifications to an airplane come with drawbacks. They can be as simple as a bit of added weight and a logbook entry or complicated enough to require a lengthy supplement to the paperwork. The more we alter the airplane’s original performance, the more we need to understand the mod’s impact on the airplane’s other characteristics. When we add more power to an airplane, for example, we also burn more fuel. What can happen to an airplane’s “book” fuel burn after a very modest power boost is this month’s lesson.
Background
On September 24, 2010, at 2249 Eastern time, a Cessna 172K collided with a tree and the ground during a forced landing in the vicinity of Chatsworth, Ga. Visual conditions prevailed. The commercial pilot in the right front seat and the private pilot in the left front seat were killed. The rear-seat passenger received serious injuries. The airplane sustained substantial damage.
The flight departed an airpark in New Smyrna Beach, Fla., picking up its IFR clearance while airborne, at 1908, en route to Chattanooga, Tenn.
At 2242:23, the pilot canceled IFR. Two minutes later, the pilot informed the controller he was landing at Dalton, Ga. Less than three minutes after changing his destination, the pilot stated, “…and Chattanooga Approach, Mayday, Mayday, Mayday. We have an engine failure.” The controller asked the pilot at 2246:00 if he was able to make the airport. He replied, “We are not sure.”
Two witnesses near the crash site observed the airplane fly directly over them. The engine was whistling like a fan. The airplane struck the top of a tree, descended nose-down and collided with a ditch/culvert. It then bounced, went through a barbed wire fence, spun around and came to rest adjacent to a road. The crash debris line extended 189 feet.
Investigation
The rear-seat passenger stated he was asleep for most of the flight. When he woke up, the pilot informed him, “I think we need to land.” The passenger looked at one of the fuel tank gauges and it was very low. The engine then started running rough. The pilot switched fuel tanks and the engine quit. The pilot declared an emergency and made a right turn. The airplane clipped a tree and the passenger later woke up in the hospital.
Despite major damage to the wings and fuselage, the propeller remained attached to the crankshaft flange. The throttle was in the full-open position, the mixture was full rich and the carburetor heat was off. The fuel selector valve was in the both position. Continuity of the flight controls was confirmed from the control yokes aft to all flight control surfaces.
The right magneto developed spark at all ignition towers when rotated by hand. The left magneto plastic points assembly was damaged and the ignition towers would not produce spark. The fuel strainer remained attached to the firewall and fuel was present in the filter screen below the fuel strainer pick-up pipe. The mixture lever was not located. No fuel was present in the fuel lines or carburetor.
The top and bottom spark plugs were removed and exhibited light white combustion deposits. The four bottom spark plugs exhibited lead fouling. Electrode wear was advanced on all. The engine was rotated by hand at the propeller flange. Compression and suction was obtained at all cylinders. The rocker arms and valves moved when the crankshaft was rotated. The oil filter was removed, opened, and was found to be free of contaminants.
Both fuel caps were intact with a tight seal. No browning of vegetation was present. Two and one-half gallons of fuel were in the left fuel tank. A quart of fuel was found in the right fuel tank, which was not ruptured.
The airplane was modified with Supplemental Type Certificate (STC) SE3692SW, which increased the engine’s maximum power rating to 160 hp. The 172 had one 21-gallon tank in each wing, of which two gallons are unusable. Review of the pilot’s flight plan revealed he had planned on a fuel burn rate of nine gph. The planned fuel burn for the flight was 33.5 gallons, but it did not take into account the fuel used by the STC’d engine of 11 gph. This was despite the airplane’s owner later stating the pilot was aware of the increased engine horsepower and fuel burn rate.
The total flight time was 3 hours and 41 minutes. The distance between the departure airport and the accident site is approximately 412 nm. At 82-percent power, the engine’s fuel burn rate is about 11.25 gph. At this fuel burn rate, 39.375 gallons of fuel would be required during the cruise portion of the accident flight.
Probable Cause
The National Transportation Safety Board determined the probable cause of this accident to include: “The pilot’s inadequate flight planning and in-flight fuel management resulting in a total loss of engine power due to fuel exhaustion. Contributing to the accident was the operator’s failure to ensure aircraft records pertaining to engine modifications and fuel burn rates were available to flight crewmembers.”
It’s always the pilot’s responsibility to ensure sufficient fuel is aboard for the planned flight. But when an aircraft’s engine has been modified to put out more power, that routinely means fuel consumption will increase. The paperwork required to be aboard the flight, detailing the additional fuel burn, apparently was available to the pilots, but it’s clear they planned the flight using the airplane’s original data.
Presuming an unmodified airplane, this was a max-range flight, leaving minimal IFR reserves. The planning was accurate right up to the point the pilots failed to use the appropriate data, reflecting the higher-power engine. Regardless of which engine was installed, an en route fuel stop would have been prudent.
