Understanding Aircraft Maintenance

I have been a Maintenance Check Pilot for quite some time and have returned many helicopters back to service after heavy maintenance.  For example, I might do all of the maintenance check flights following a Robinson R44 overhaul or AS350/H125 12-Year inspection.  In both of these cases, the aircraft was stripped down to its bare bones and then rebuilt.  It is obvious that a thorough inspection before flight is necessary. 

Many aircraft manufacturers have established maintenance test procedures published in either the Rotorcraft Flight Manual (RFM or POH) of the aircraft’s maintenance manual.  This is the first area that can be confusing for someone returning an aircraft to service after maintenance.  Let me start with, perhaps, an obvious point that gets overlooked by many pilots.  The person who performs the maintenance on the aircraft is likely an Airframe and Powerplant mechanic.  As pilots, we sort of lump all mechanics as A&Ps without truly understanding the difference. 

Commonly, an aircraft mechanic holds both airframe and powerplant mechanic authorizations but they are different certifications.  You may find a mechanic who holds only an Airframe or only a Powerplant Mechanic’s certificate.  These certifications require different written and practical tests. 

Similarly, aircraft are usually divided into Airframe and Powerplant components.  Let’s look at the Robinson R22, for example.  The Robinson R22 airframe is, by most accounts, the same throughout its series.  The HP, Alpha, Beta, and Beta II airframe is largely the same and requires very similar maintenance.  However, the engines can be significantly different.  Not to mention, Robinson Helicopter Company is not the manufacturer of the engine; Lycoming is.  As a result, there are maintenance procedures for the airframe that are developed by Robinson Helicopter Company and engine maintenance procedures that are developed by Lycoming.  These procedures are found in different manuals. 

We can see, already, that maintenance checks require understanding what component is to be tested.  To complicate things even further, many aircraft have Supplemental Type Certificates or accessories that require maintenance that isn’t spelled out in the airframe or engine maintenance manual.  These are often found with the STC or Instructions for Continued Airworthiness (referred to as ICAs).

Unfortunately, the Airmen Certification Standards and most pilot school syllabi do not really dive deep enough into understanding aircraft maintenance.  In effect, the ACS requires the pilot to understand basic concepts in 14 CFR Part 43 and Part 91; such as what inspections are required and who can accomplish them.  I have yet to find a syllabus that discusses ICAs, Form 337 Field Approvals, or accessories.

So, let’s get back to what it means to understand aircraft maintenance.  I will start by sharing a brief story.  I was recently in the field when I needed to participate in a compressor wash of a turbine engine.  For those pilots that don’t have turbine time, this is simply an internal cleaning of engine components.  I have, for years, helped complete compressor washes in various helicopters.  It was my first time, however, assisting with a compressor wash on this particular helicopter model.  Unfortunately, I didn’t know we needed to do this procedure until it was late in the day and I still had to do a debriefing with the crew.  By the time the mechanic was ready for me to do my part, I hadn’t really had time to review the procedure.  As a good maintenance check pilot (although a compressor wash isn’t really considered a maintenance test procedure), I asked for the procedure from the ENGINE manual so that I can be sure I do my part correctly.  In cases of compressor washes, there is often a specific time when the solution must be introduced to the engine and how long I can motor the engine.  In any case, I reviewed the procedure and had a few questions.  While the mechanic answered my questions, I still had doubts but followed his lead.  As it would turn out, my doubts about the procedure were correct.  I, later that evening, learned we had not done the compressor wash correctly.  In a twist of luck, we hadn’t hurt any internal components and the method we used was sufficient; albeit not correct.

Simply relying on the mechanic to share with us his or her knowledge is not always the best way to go.  This isn’t to say that we shouldn’t or can’t trust our mechanics but it is our responsibility to verify.  At the end of the day, we are going to be stuck with whatever maintenance they perform.  This is one of the main reasons why it is imperative for pilots to understand aircraft maintenance and maintenance procedures.  Equally important is our understanding of the systems that require maintenance.  Please allow me to share another example.

A few years ago, I was helping return an R44 back to service after a 100-hour inspection.  Several mechanics had been involved in the process but only two were responsible for the finishing touches and I was asked to help them.  My main purpose was the help with the Track and Balance of the main rotor system.  While we were letting the engine warm up, I elected to run through a series of checks that are usually reserved for the Annual Inspection or overhaul.  During that test, I discovered that the engine and rotor tachometer would discontinue working when I turned the battery and alternator switches off.  My basic knowledge of the aircraft systems (and emergency procedures) indicated that this was incorrect.  When I inquired about this with the senior mechanic, he quickly recognized there was something not right with the helicopter.  He diagnosed the problem quite efficiently and we were back in the air within the afternoon.  What bothered me was that this aircraft had recently undergone an Annual Inspection and this was not discovered by the mechanic or pilot who helped out.

There are a few things that we can do to better understand that maintenance required on our helicopters.  This will help us understand what happening with our aircraft more clearly.

Although it seems obvious, we should spent time in the maintenance manuals for the aircraft we fly.  It is acceptable that we do not understand everything in those manuals but we should make a concerted effort to study them and develop questions about the procedures, what mechanics are looking for and how they accomplish the tasks laid out in the manual.  When I know an aircraft is about to undergo a maintenance procedure, I try to find that procedure in the appropriate manual and read through it.  Unlike many pilots, I prefer the text part of the procedure rather than the photos of the procedure.  I like a step-by-step description of what is going to happen and how.  That doesn’t mean I don’t look at the illustrations, however.  I study the illustrations so that I can better understand the components.  Just recently, I studied the illustration for a Drive-Link and Scissor assembly on a Bell Helicopter and was astounded by the number of smaller components that are part of that assembly.  It had more bearings and spacers that I originally expected.

Maintenance manuals are regularly updated but there are other publications that involve maintenance.  As pilots, we are generally aware of Airworthiness Directive and Service Bulletins.  Let’s examine Lycoming’s Service Bulletin 388, which is commonly referred to as the “Valve Wiggle”.  In Robinson Helicopters this is done every 300 hours or anytime the valves are suspected of sticking.  I would venture a guess that many, if not most, Robinson pilots have heard of this inspection but how many have actually read through the procedure.  This 13–14-page bulletin explains the tools used, the method to test, and alternate methods to accomplish this inspection.  Not to mention, it has some pretty great illustrations.  Ask your maintainer which Service Bulletins he or she has on your aircraft and if you can get a copy.  You’d be surprised what you can find.

Utilize a Post-Maintenance Checklist.  The company I fly for has a Post-Maintenance checklist on which every mechanic writes down what was done during a maintenance procedure.  For example, the mechanic will write the following:

Removed and replaced left-hand magneto.

When all of the maintenance is completed and the aircraft is returned to the flight line, this checklist is left for the pilot to review as part of the preflight inspection.  This serves two functions.  The first, the pilot will now know what items s/he must pay particular attention to when inspecting the aircraft.  Secondly, the pilot can be better informed if something out-of-the-ordinary happens in flight.  This is a good way to ensure the mechanic hasn’t accidentally left a tool on a transmission deck, a rag near an intake or forgotten to replace some screws.

When I can, I ask the mechanic who completed the work to walk me through what was done.  I ask the mechanic to show me the parts and pieces on the helicopter that had maintenance completed.  This gives me a chance to ask questions and get a stronger understanding of the maintenance performed.  This leads me to the next part, CHALLENGE.

As I allow the mechanic to explain what was done, I ask probing questions.  For example, if the mechanic tells me that he torqued a bolt, I will ask “to what torque value?”.  During a recent maintenance procedure, the mechanic told me that he replaced some fuel lines and I asked him if there was a required torque.  While I didn’t know the exact torque value for that particularly B-nut, I knew it required a specified torque.  I was almost shocked when he replied “I tightened them back to the previous torque stripe.”  Let me be clear, this is a method some mechanics use but it isn’t one I prefer.  I asked him to look up the torque and check each nut.  No surprise, the torque was correct but I wanted to verify.  There was a fatal AS350 accident in 2010 in which the main fuel line disconnected from the engine because it was not properly torqued.  There is a lot more to this story that I will hit on it a bit. 

If your company and mechanic allow, participate in the maintenance procedure.  This doesn’t mean that you have to be the person doing the work, but you can get involved.  Sometimes this is as simple as cleaning parts or handing tools to a mechanic.  A few years ago, I was on a USFS firefighting contract and we had to replace a part on an H125.  The procedure was quite simple and the mechanic told me he didn’t need any help, so I asked if I can watch.  He didn’t mind and, when he was done, I learned more about the component he was replacing.

Maintenance is more than turning a wrench.  Ask any mechanic and they will tell you that the paperwork is just as complicated and important as the procedure itself.  Not only does the mechanic have to make a logbook entry, they often have to tag parts, calculate times, and change weight and balance.  When I am asked to return an aircraft to service, I could spend an entire afternoon reviewing the paperwork.  There have been times when I scrutinized every maintenance entry and work card.  The aircraft is only as safe as the maintenance performed and is only as legal as the paperwork. 

A few years ago, we added a helicopter to our fleet.  After the mechanics completed an Annual Inspection, I asked to go through the paperwork and weight and balance.  When I ran a quick weight and balance for a normal configuration of the aircraft, it was forward of the Center of Gravity limit.  This was very unusual.  I scoured the list of installed equipment only to discover that the arm listed for a few of the components was incorrect.  We made the correction and the aircraft was, in fact, well within limits.  This goes to show that a detailed look into the paperwork is just as important as the work itself.

So let me get back to the fatal accident in 2010.  N509AM was operated by Air Methods and had undergone normal but substantial maintenance.  One mechanic was responsible for completing and inspecting the maintenance (this could be a whole topic unto itself).  A pilot was assigned to complete the post-maintenance check flight.  The flight lasted less than eight (8) minutes and the aircraft was returned to duty.  On the very next flight, the aircraft was near maximum gross weight with a pilot and two medical crewmembers.  While enroute, the engine had a catastrophic loss of power and the pilot attempted an autorotational landing.  Unfortunately, the aircraft impacted terrain and was consumed by a post-crash fire.

Investigators discovered that the main fuel line had become disconnected from the engine causing a fuel starvation.  The engine had not failed but did exactly what it is supposed to do when fuel is removed.  It quit.  The failure rests with the mechanic and pilot who put the aircraft into service.  I want to be careful because I don’t anticipate they willing endangered the lives of the crew but they let some basic principles slip by. 

If we look at this accident from the perspective of the maintenance check pilot, we can see a few glaring missteps that could have prevented this tragedy.  The AS350 flight manual has several Test Sheets that are to be used after certain maintenance is performed.  In the case of the accident aircraft, major modules of the engine had been replaced.  A review of the Test Sheets indicates several in-flight tests to be performed.  I have, in fact, completed these very same tests.  The NTSB concluded that these tests should have taken somewhere between 30 and 45 minutes; I agree.  The 7.5-minute flight conducted by the maintenance check pilot was not sufficient in length or test capability to adequately determine if the engine was suitable for return to service.  Taking the time to study the maintenance procedures and test procedures would have, likely, caused the engine to quit during the testing.  I would argue that there is no good time for an engine to quit, but an engine stoppage on a test flight is far better than an engine quitting on “the line”.  Hopefully, when a pilot is conducting a maintenance check flight, s/he is prepared for the engine failure and has put the aircraft in a position for a successful outcome.  When the engine actually quit, the line pilot was not in a position or prepared.

Secondly, there is very little evidence that the pilot reviewed the maintenance procedure in any notable way.  If the maintenance pilot had spent any time learning about the maintenance procedure and what was done, there is a chance he would have spent extra time inspecting the main fuel line to the helicopter.  When looking at the maintenance manual, you can clearly see that the main fuel line is connected to the engine with two bolts and a B-nut.  My guess is that the pilot only inspected the B-nut, which was secured to the engine.  The union from the B-nut to the engine casing has two bolts that hold it into place.  Those two bolts are what came loose.  The engine maintenance manual details this procedure in text and in illustration.

Finally, a few probing questions might have revealed that the installing mechanic had not tightened the bolts from the union to the engine casing as required by the maintenance manual.  If the maintenance check pilot has asked the mechanic which bolts had been torqued and to what value, the mechanic may have realized he did not complete that part of the maintenance task.  A good post-maintenance checklist would have gone a long way.

Because the accident had occurred, it is easy to see how all the “lines in the cheese” lined up but, in the moment, it can be hard to notice.  One of the best ways to prevent the accident is to truly understand how maintenance is performed, when it is performed and how you can be involved.  It is for these reasons that pilots must be informed, aware, and knowledgeable.