When you're told to replace them! Good safe hoses are replaced everyday on aircraft because of regulatory requirement or maintenance policy. But, absolutely unsafe, un-airworthy, nasty hoses continue in service year after year because of unclear guidelines. Here's a more holistic approach to hose evaluation and replacement:
Hose replacement is based on:
All three should be considered, none should be ignored.
Policy and regulations are often based on time or cycles and ignore condition. Typically these policy's or regulations are written in a manner that implies that it's the only criteria used to replace hoses. For example, one manual reads: "Replace flammable fluid carrying rubber hoses every 5 years". As the mechanic strictly follows the FAA's dictate to "follow the manual", the only inspection performed on these hoses becomes a time inspection. Hose inspection and condition evaluation is not performed or diminished in importance. Conditional inspection must be part of any hose replacement policy.
Unfortunately, Instructions for Continued Airworthiness are often vague, incomplete, or missing when it comes to hose inspection criteria. Here are some guidelines for hose replacement; some tongue-and-cheek, and some very serious. This is not a complete list. just some examples. Enjoy!
Hose should be replaced when it has a hole in it. This was on the right engine - go take a look at the same hose on the left engine! If 3 wire braids are broken -replace the hose. Chafing is always out-of-sight, under, behind, or covered. Your hand is often the best tool. Feel behind the hose where its resting against something.
The hose should be replaced when you can break it in half. This hose could have been broken in half the last 10 annual inspections. Notice how the corrosion is somewhat hidden under the jacket. If the hose makes crunching noise when you move it --it's time to replace.
Hose should be replaced when the firesleeving fills up with fuel. If something smells foul - you have to do something about it. Your nose is also an inspection tool - both for the mechanic and pilot. If your engine compartment smells like fuel - something is amiss!
Cyclic fatigue failure after 12,369 hydraulic cycles. For hydraulic hoses on high-usage aircraft, pressure or movement cycles need to be considered in your hose replacement policy.
Hose should be replaced when it is evident that it was improperly assembled. This one gets overlooked each time. You know what the problem is here? Those undersized mandrels they sell to mechanics. They don't fully compress the rubber bulge so the nut/nipple gets real hard to turn. This guy just gave up when the going got tough. See my article on Battle of the Bulge
. Oh, yea, remove this hose from service - it's not put together right. Proper distance between the nut and socket for this style of hose (111, 303, MIL-dtl-8794) is:.005 to .031 inch.
Hose rapped in oven-rap foil. What was this engineer thinking? Hose cannot be inspected if shrouded in aluminum foil. Yet the mechanic was blamed for not inspecting the hidden hose underneath!
Similar hose with hole after the fire burned off the oven-rap aluminum foil. Loss of Enstrom F-28 C, G-WSEC
AAIU Synoptic Report No: 2008-017: A contributing cause of the accident was "Maintenance of the helicopter had failed to detect that significant fretting of the fuel hose was taking place over an extended period of time."
Engineers need to understand the conditions and capabilities of maintenance, and design their products to be maintainable under those conditions. The responsibility for hose inspection is a joint responsibility involving the engineer, the mechanic, and the regulatory agency. Any or all can be a contributing cause to a hose failure.