Differential Diagnosis

Sep 1, 2016

Resources > Magazine Articles > Differential Diagnosis

Fixing is usually the easiest part of aircraft maintenance. Figuring out what’s wrong is usually the hardest part. 

By Mike Busch | A&P/IA

A funny thing happened on my way to Milwaukee…

It was 2013 and I was flying my trusty 1979 Cessna T310R to speak at the annual national convention of the Flying Physicians Association. My talk to the flying docs was on the subject of troubleshooting. Little did I know that my troubleshooting skills were about to be put to the test.

I went wheels-up from my home base in Santa Maria, California, about 8 AM, headed for Denver’s Front Range Airport. The forecast called for thunderstorms after 1 PM, and my plan was to arrive early enough to miss them. The plan worked…although less than an hour after my arrival, a tornado touched down about 5 miles north and shut down Denver International Airport for a while.

The next morning, all was calm at Front Range as I taxied out for takeoff, destined for Milwaukee’s Timmerman Airport. I was cleared for takeoff, released the brakes, smoothly advanced the throttles to the stop and started my takeoff roll on runway 17. 

Uh oh! Something felt wrong. It was taking way too much right rudder to hold the centerline. I scanned the engine gauges and noted a big split in the manifold pressure needles. The right engine was showing 32 inches as it should, but the left engine was only 24”. Not good. I retarded both throttles to idle, advised the tower I was rejecting the takeoff, and pulled off the runway at the next intersection.

I taxied back to the runup area and tried a full-power runup with the brakes set. The same thing happened: 32 inches MAP on the right engine, 24 inches on the left. It was reproducible. A moment of reflection revealed that 24 inches is roughly what one would expect from a normally-aspirated engine on takeoff from a field elevation of 5,500’. 

Hmmm… It appeared that the left engine was operating normally-aspirated. Its turbocharging system had stopped working, seemingly overnight. Why? I didn’t really have a clue.

What the @#$% could be wrong?

As I taxied back to the ramp in search of a shop where I could borrow some tools, I started making a mental list of all the things I could think of that could produce these symptoms. (Making such a list should always be the first step in troubleshooting.) Visualizing how the turbosystem works and considering its various failure modes, I came up with six possibilities:

  1. Failed turbocharger.
  2. Stuck-open wastegate.
  3. Failed controller.
  4. Big induction system leak.
  5. Big exhaust system leak.
  6. Big oil leak in controller/wastegate system.

I taxied past the open door of a hangar inside which I saw two guys in work suits wrenching on airplanes, each with a big red Craftsman tool cabinet. I shut down the engines, climbed out of the airplane, introduced myself, and explained my predicament. They seemed happy to help.

My first step was to remove the top cowling from the left engine and look around for something obviously wrong. I saw no signs of oil or exhaust stains where they didn’t belong, so I crossed items #5 and #6 off my suspect list. I went over the induction system with a flashlight and mirror and couldn’t see anything untoward, so I crossed off item #4, too.

I popped open the cover of the induction air filter canister, removed the filter element, and inspected the turbocharger’s compressor. It looked pristine, with no hint of foreign object damage. I reached in and spun the rotor with my fingers, and wiggled it to check its radial and axial play. It turned freely and felt normal. I crossed item #1 off my list.

Isolating the fault

Now I’d whittled my list of six suspects down to two likely culprits: the wastegate and the controller. I spent a few minutes thinking about how I could figure out which one was the bad guy. I came up with a plan: By removing the oil return line from the controller to the engine and capping it off, I could effectively disable the controller and force maximum oil pressure to the wastegate actuator. If the wastegate was working, then this should result in a fully closed wastegate and maximum turbo boost. If red-line MAP still wasn’t available, that would prove that the wastegate was bad.

We rummaged around the toolboxes until we found some suitable AN caps to cap off the oil line. I reinstalled the top cowling, climbed into the cockpit, and started the left engine. When I advanced the throttle, MAP climbed up to the red-line at 32 inches. I grinned, then shut the engine down and climbed back out of the cockpit.

I’d just proven that the wastegate was fine. In fact, I’d proved that all the turbosystem components were fine except for the controller (which I’d disabled). It was now the only remaining item on my list. By the process of elimination, it must be the culprit.

Fixit time

This was both good news and bad news. The good news was that I was now confident I knew what was wrong: the controller was malfunctioning. The bad news is that it would take at least 48 hours to get a replacement controller overnighted to me in Denver, and by then it would be too late to make my speaking engagement in Milwaukee. It was looking like I might be forced to abandon my trusty Cessna in Denver and (gasp!) catch an airline flight to Milwaukee, then return to Denver on the airlines and replace the controller then.

Ugh! I really didn’t want to do that…

In an attempt to avoid this unspeakable fate, I started giving serious consideration to removing the controller from the airplane and disassembling it, hoping I could figure out what was wrong with it and maybe even coax it into working. This is not something an A&P would normally do as it’s considered a highly specialized procedure. But I figured since the controller was already broken, I had little to lose by taking it apart.

Another thought occurred to me: What if the only problem with the controller was that it had become contaminated with some sort of debris that got stuck in its poppet valve and prevented it from closing? If the valve couldn’t close, the controller couldn’t work. That would account for the symptoms I was seeing. I wondered whether there might be a way of cleaning any debris from the poppet valve without taking the controller apart. 

I had a sneaky idea: I disconnected both oil lines from the controller, and asked my new mechanic friends whether I could borrow their air compressor for a few minutes. Using a rubber-tipped air nozzle, I hit the controller’s oil output port with several shots of 80 PSI air while holding a shop rag over the oil inlet port to catch any expelled oil. My idea was to “backflush” the poppet valve with an air blast and hopefully dislodge any debris that might be stuck. Sure enough, some flakes of what looked to be carbonized oil wound up in the rag.

I reconnected the oil lines, reinstalled the top cowling, climbed into the cockpit, murmured a silent prayer, and started the left engine. After letting it warm up for a few minutes, I slowly advanced the throttle while watching the MAP gauge. The needle advanced smoothly up to…YESSSS!!!…32 inches.

I said my goodbyes to my mechanic friends, re-filed my IFR flight plan from my iPhone, and taxied out for takeoff. Everything worked as advertised, and I arrived at Milwaukee only two hours behind my original schedule. Upon checking into my hotel room, I pulled out my notebook computer and revised my PowerPoint presentation to the flying docs, because the story of my little turbosystem adventure was just too apropos not to share.

Differential diagnosis

What I just described to you is a textbook example of what doctors call “differential diagnosis.” To quote Wikipedia:

“Differential diagnosis (abbreviated DDx) is a systematic method of distinguishing a disease or condition from others presenting with similar symptoms. This method is essentially a process of elimination that excludes candidate conditions until a single probable diagnosis remains.”

Every physician receives extensive training on this technique in medical school. It typically involves five steps:

  1. Gather information about the symptoms.
  2. List candidate conditions consistent with these symptoms.
  3. Prioritize the list of candidate conditions.
  4. Rule out candidate conditions (through testing or therapy) until a definitive diagnosis has been established through the process of elimination.
  5. Verify that the surviving diagnosis is correct.

The application of the DDx technique is hardly limited to medicine. It is, in fact, the way all troubleshooting of complex systems should be done, whether dealing with human bodies, household plumbing, cars, boats, or airplanes.

Shotgunning and overkill

Unfortunately, A&Ps tend not to be nearly as well-trained in the DDx technique as MDs are. This is an area of aircraft mechanic training that is greatly in need of improvement. A&Ps could learn a lot about troubleshooting from their doctors. In lieu of performing proper DDx, I see a lot of mechanics using two alternate techniques that I like to call “shotgunning” and “overkill.”

Shotgunning occurs when—instead of methodically analyzing and eliminating possible failure modes—a mechanic simply replaces components on a trial-and-error basis, hoping to get lucky. If the mechanic guesses right the first time, he comes out looking like a hero. If he doesn’t, the aircraft owner often winds up passing out from sticker shock. The mechanic’s initial guess might be (1) whatever component turned out to be the culprit last time he saw a similar problem, (2) whatever component he happens to have on the shelf, (3) whatever components is easiest to replace, or (4) whatever component is most expensive. Whatever the exact algorithm, shotgunning is based on guesswork, not analysis. In my view, it happens way too often.

Overkill occurs when a mechanic elects a corrective action that goes far beyond what is required or appropriate to deal with the problem at hand. One classic example (all too common) is the A&P who finds a small quantity of metal in the oil filter and immediately concludes that the engine must be torn down. Or when a mechanic reacts to an owner’s report of increasing oil consumption by recommending a top overhaul. (Do you think the mechanic would take these costly, invasive actions if it was his own airplane and he was footing the maintenance bill?)

In my experience, shotgunning usually results from a mechanic’s lack of training and/or systems knowledge. Overkill, on the other hand, is usually prompted by a mechanic’s fear of being sued if something goes wrong, and his consequent desire to transfer the liability burden to someone else (e.g., the component manufacturer or overhaul shop).

As aircraft owners, we can and should protect ourselves from being victimized by shotgunning or overkill. It’s really not that hard. Next time a mechanic troubleshoots your airplane and renders a diagnosis that calls for something expensive or invasive to be done, simply ask him to explain to you in detail how he arrived at his differential diagnosis. (“What are the possible failure modes you considered, and how did you eliminate all but this one?”) If your mechanic pfumphers his explanation, you can bet he’s shotgunning or overkilling, and it’s time for you to seek a second opinion. 

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