May 1, 2003
By Bruce Landsberg
Bruce Landsberg is the executive director of the AOPA Air Safety Foundation.
Etomology, the study of bugs, is not something that many of us consider when we go flying. Yet insects can wreak havoc on a variety of aircraft systems, and pilots probably don't pay enough attention to the small tubes, vents, and orifices that are present in every airplane. Looking through the Air Safety Foundation database over the past 25 years, we found more than 20 accidents attributed to insects in fuel vent lines, pitot tubes, carburetors, and other sensitive places. It's quite likely that there were some unexplained engine stoppages caused by insects blocking vents.
A quick refresher on fuel system plumbing: As fuel is pumped to the engine from the bottom of the tank, air must flow through the vent at the top of the tank or a vacuum is created and the resulting suction stops the fuel from flowing. The vacuum can also cause fuel bladders, and in some cases tanks, to collapse. Fuel vents are found in a variety of places depending on the make and model. The most likely spot is the underside of the wing, but occasionally vents are located on the trailing edge (as in Cessna 210s). On some Cessnas there is also an alternate vent in the fuel cap that opens to allow air through when a vacuum is applied to the tank. On a hot day when fuel expands or when the tank is overfilled, you can see fuel dripping from the vent. While not especially good for the environment, it does show that the passage is open.
On preflight inspections, while we may give vents and tubes a close look, it's tough to really ascertain that air is flowing, especially if life forms crawl up out of view. Then the trouble starts. It appears to be more likely in the South and with relatively inactive aircraft, but accidents have occurred up north and in busy aircraft. The size of the aircraft is irrelevant. Let's look at a few of the mishaps.
While en route in a Cessna 441 at Flight Level 240, the pilot noticed that the wing deice boots were inflated. There were no icing conditions and the boots had not been activated. The pilot's operating handbook provided no guidance. While descending through 5,000 feet msl near the destination, the pilot heard a loud bang and felt the airplane jolt. The right wing caved in and aileron control was limited. The pilot declared an emergency and made an uneventful landing.
Examination of the engine bleed-air control valve overboard lines showed both were plugged with mud about 18 inches inside the overboard openings and both of the NACA under-wing fuel vents were plugged with mud about 36 inches from the openings. The right wing was structurally damaged with similar internal damage to the left wing. Three wing rib stiffeners had punctured the lower wing skin of the wet wing. Prior to the accident the aircraft was parked outside for 30 days with pitot tube and engine inlet covers installed. Rags were also stuffed in the tailcone area openings to keep birds from nesting, but there were no protective covers provided for the NACA under-wing fuel vents nor for the engine bleed-air overboard lines.
The aircraft had been partially repainted to change foreign registry numbers on top of the wings, and fuel filler caps from both wings were sent to the manufacturer for testing. The right-wing filler-cap secondary vent was reduced by 50 percent because of paint blockage. The local college entomology department found evidence of mud dauber wasps in both the fuel vent and bleed-air lines.
According to the 441 maintenance manual, the fuel caps are to be inspected for damage every 200 hours or 12 months, whichever occurs first, but there was no detailed information on what to look for in the maintenance data. There was also no information found addressing the vent holes or their sizes, or tests for the flutter valves.
A Yakovlev 52, piloted by an ATP, suffered engine stoppage only three-quarters of a mile from the destination airport and this after 90 minutes of flight. The fuel tanks were topped prior to flight and all was routine until landing pattern entry. The engine "sputtered to a halt" from what seemed like fuel starvation, although the pilot knew he still had 20 to 25 minutes of fuel on board. The aircraft landed short in a plowed field and flipped over.
Examination revealed that the right fuel tank was empty, but that the left fuel tank still had fuel. The main fuel vent line was clogged about 4 to 5 inches from the vent opening by a mud dauber nest that would have been impossible to see during a preflight inspection. The inspector noted that there was difficulty in removing the fuel tank caps for fuel tank inspection because of the vacuum created by the clogged vent line.
Now let's move from where the aircraft's performance is actually degraded to where it just appears to be degraded. A blocked pitot tube shows up as no airspeed or as sub-normal speed. Usually this happens during takeoff when time to resolve the issue is short.
A Piper Cherokee pilot aborted takeoff halfway down a 3,600-ft runway after noting the airspeed indicator was inoperative. The aircraft slid off the end and into a ditch. Investigation showed a pitot-head blockage by mud daubers that was not visible from the exterior. A local A&P mechanic noted that the problem was common at the airport and, even though this aircraft had flown only a few days earlier with no problems, the pitot tube cover was not used following that flight.
A Cessna 182, flown by the private pilot owner and a CFI, also ran off the end of a runway during an aborted takeoff at the beginning of an instrument instructional flight. Both pilots reported that the preflight inspection and engine runup were normal. The CFI noticed nothing unusual until the airplane was about one-third down the runway, and "had not yet begun to lift off ... at this point where the airplane is usually breaking ground. The airspeed indicator was indicating zero."
The private pilot stated that he reduced the throttle and began braking to abort the takeoff. The CFI reported she assisted in brake application and keeping the airplane under control. The airplane ran off the departure end of the 3,200-ft runway, continued up a small hill, and came to rest in a cornfield.
Post-accident examination revealed a mud dauber nest blocking the pitot tube. There was no evidence of pre-impact mechanical malfunction. A pair of heavy, black skid marks, approximately 600 feet long, continued down the runway and off the end to where the airplane came to rest. The airport manager witnessed the airplane as it ran off the runway, and he estimated its speed to be 40 mph. He stated that both pilots were "standing on the brakes, with full power applied, for about 1,000 feet.... They finally throttled back near the end of the pavement."
A few observations regarding takeoff: Many pilots have takeoff expectancy — they expect the airplane to fly and it usually does. But the pro tries to think through what nastiness might be in store and is pleasantly surprised when something doesn't go wrong. Looking and listening to the airplane are mandatory. A quick scan of power and airspeed instruments early in the takeoff run shows if indications are normal. If not, that's a great time to reconsider the takeoff. Naturally, airplanes stop much better if you remove power rather than applying brakes against an engine at full bore. Brain fade in such circumstances is not uncommon, which is why thinking about a possible rejected takeoff just before taking the runway is great preparation.
Airspeed indication problems are best resolved on the ground, but if you are too far down the runway and the aircraft is performing normally, power plus proper pitch equals performance — the airplane will fly. Set up for a normal landing with flaps, gear, and power where they normally are and you should have an uneventful arrival.
So, even a thorough preflight may not solve the bug problem. Prevention is the only way and that means using covers for pitot and fuel vents. It also means being darn sure to remove them — in the case of the fuel vents, the engine will likely run just long enough to put the aircraft in a very compromised position either during or just after takeoff. Being bugged just isn't a good thing in an aircraft.
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