This year marked my twenty-fifth pilgrimage to Oshkosh, Wisconsin, for general aviation's annual weeklong binge on airplanes and everything that makes them tick. However, our trip wasn't without incident.
Upon landing, I opened up the cowl as part of my post-flight inspection and discovered blue staining on the upper cowl, oil filler cap, and a few other items on top of the engine. Blue staining on an aircraft means only one thing: uncontained fuel. Uncontained fuel in a hot engine compartment can lead to fire. We were very lucky to be on the ground safely. Now we had to diagnose and repair the leak.
Flexible hoses for oil and fuel on aircraft are available in either rubber or Teflon varieties. The hose assembly is made of up the inner tube, the reinforcement, and the covering. The covering can include a separate fire sleeve, like the one on our original hose, or an integral molded fire sleeve, commonly found on custom Teflon hoses.
Although legal, the Type C rubber hose that we removed isn’t ideal for use in an engine compartment. These hoses are rated for only 250 degrees Fahrenheit and are protected by the fire sleeve that slips over the hose and is clamped at both ends. There is an airworthiness directive for oil cooler hoses on Piper aircraft that requires repetitive 100-hour inspections on Type C hoses. Type D hoses are not subject to the 100-hour inspection, but still have life limits recommended at 8 years or 1,000 hours by the FAA. Teflon hoses are not life limited per-se, but all hoses should be inspected regularly for condition. Every manufactured hose has a data tag that includes critical information about the hose, including the hose type and date of manufacture. It’s often seen as a code with the quarter and year of manufacture (1Q14 = first quarter of 2014).
I strongly recommend inspecting all flexible hoses in the engine compartment frequently and replacing Type C and D rubber hoses if they are stiff, chafing, or show signs of wear. If you have a question, remove the hose and try to flex it. If you hear any cracking sounds, replace it. I’ve seen rubber hoses fail internally well before they reach 8 years old and it’s not always easy to detect when installed.
Case in point, one of my close friends, Jay Drury, is a highly regarded A&P/IA. He was recently working on a customer’s aircraft that had been having issues with high oil temps. He traced the issue to a flexible rubber hose that had failed internally, closing off the flow of oil to the oil cooler when the engine compartment got hot—exactly the time you don’t want to stop oil from getting to the cooler. From the outside, the hose looked normal, but the inner layer of rubber was collapsing inward, causing the blockage. The failure was only visible with the hose removed, but the stiffness in the hose was a sign that it was reaching the end of its life.
At our post-landing inspection at Oshkosh, we removed the failed hose and tied the airplane down for the week. The next day, we visited with Jack Samuels from Aircraft Spruce and Specialty Co. who saved the day, delivering a beautiful new Teflon replacement hose assembly to us in time to get it installed before we needed to depart for home. You can watch a video of our 2018 EAA AirVenture hose adventure online.
Flexible hoses on aircraft engines are vital to moving oil and fuel where it needs to go, but it’s critical that the fluids stay contained around a hot engine. So, inspect your hoses regularly, know their age, and replace them as soon as they show signs of fatigue, heat, or damage. Until next time, happy flying!