Lost Knowledge

During any restoration project, an effort is made to learn what was known when the airplane was originally built and flown. Sometimes pieces of information are lost and must be relearned. With aviation being very unforgiving to carelessness, sometimes the results are catastrophic. 

On October 1, 1940 NC97W was flipped onto it’s back at Monache Meadows California. The only reference in the aircraft logbook is: “Oct. Cracked up at Monache by George”

While researching documents during the restoration process, a factory drawing marked PA 7 was located that describes a four-inch extension tube which was placed between the carburetor and the engine inlet – effectively lowering the carburetor by four inches. Hmmm….. Why would Pitcairn do that?

The Pitcairn PA 7 has a fuselage mounted fuel tank. When placing the airplane in a nose high climb attitude, the bottom of the tank, where fuel is fed from, measures 12” above the carburetor inlet (This is with the 4” extension attached). Reading the Wright R760 Instruction Book, the minimum “head” is listed as 20”.  So to achieve the minimum head there would need to be at least 8” or 20 gallons of fuel in the tank. To achieve the required head pressure if the 4” extension was not installed would require about 40 gallons of fuel in the tank. By todays standards, fuel below the minimum level would be classified as “unusable fuel” and not considered for flight planning purposes. Of course, in 1930, these standards (which are now detailed in FAA Part 23 rules) did not exist. 

After talking with a person who is familiar with the airstrip at Monache Meadows, and showing him the pictures of NC97W upside down, it was speculated that this was probably a takeoff accident and not landing. 

After installing the 4” extension and all the cowling pieces back onto NC97W there is a set of “witness” marks where the cowl had previously been rubbing onto the air intake box. These lines seem to indicate that the air intake box was located 4” higher than it is now with the extension installed. A close look at the above photo confirms this fact.

Monache Meadows is about 8,000 feet in field elevation, making the air thin and resulting in poor airfcraft and engine performance. NC97W was used to haul hunters, fishermen, heavy gear, and more to and from this field. Any pilot when departing Lone Pine for this round trip would only carry enough fuel to fly the mission plus reserve. When departing Monache for the return trip, the fuel level would typically be at the minimum required, just enough to get down off the mountain and back to Lone Pine, plus reserve.

The flight is about 30 minutes each way, which equates to 16 gallons burn, plus 8 gallons reserve. This puts the minimum fuel level departing Lone Pine at 24 gallons and the fuel departing Monache at 16 gallons. Oops, 20 gallons is needed on takeoff to fulfill Wright Whirlwind Engine requirements WITH the 4” extension installed! Without the 4” extension, another 24 gallons would be needed in the tank. That is quite a lot of fuel to tanker from Lone Pine and to then depart Menache with, which would have reduced the useable payload needed for hunters and their gear. 

Back in 1937, when the airplane was damaged in Burbank, the engine was removed and the 4” extension probably went with it. In 1938, the current engine was purchased and would not have had the 4” extension, as that is a Pitcairn part. How would the mechanic who installed the new engine know that the extension needed to be there? Lost knowledge regained!

Did NC97W simply lose fuel flow and power on takeoff or initial climb as a result of not having the 4” extension installed? We can never really know for sure. Many biplanes and high wing airplanes have their fuel tanks mounted above, providing ample head pressure to feed fuel. However, many have fuselage mounted tanks like the Pitcairn that require special attention. How many accidents have resulted from a simple oversight?