In the early days of the King Air and PT6 engines, back in the ’60s and early ’70s, there was no such thing as chip detectors. The low spot near the bottom of the reduction gearbox (RGB) at the front of the engine had a drain plug only. The plug was not fitted with any device that would check for metal particles in the RGB housing.
But a few of those early PT6s had RGB failures that led to the power turbine (PT) being “uncoupled” from the propeller. Realize that all of the various propeller governors are connected to the output shaft of the RGB – the shaft to which the propeller is bolted. Nothing directly monitors the speed of the PT, the input shaft. So, when the disconnect occurs, the PT has almost no rotational resistance and it runs away to extremely high speeds, causing the turbine blades on the PT disk to be “liberated.” That’s a fancy way of saying that the PT catastrophically fails, sending some of the PT blade fragments out through the exhaust stacks (Wow! What an expensive sparkler show!) and also sending some blade fragments right through the engine casing. These fragments sometimes made their way through the fuselage skin and even occasionally into the crew’s legs. Yuck!
The first step taken to deal with this obvious weakness was to add the PT containment ring that all PT6s now have. This heavy band of metal surrounds the PT disk – or disks, in the larger models that have a two-stage PT – and prevents the liberated blades from exiting tangential to the axis of rotation. Now the exhaust stack sparkler show is even more impressive!
The second step was to add the chip detector, so that the pilots could be informed that the RGB was “making metal.” The detector is quite simple, comprised of two magnetic probes close together but not touching. If ferrous metal (ferrous means iron-based, which is an electrical conductor) bridges the gap, then the completion of the circuit advises the crew that all is not right in the RGB’s world.
Somewhat surprisingly, when chip detectors were first installed, no annunciators were associated with them. Instead, the detectors had to be tested with an ohmmeter every 25 hours. If continuity was discovered, then further flight was prohibited until the RGB and the engine filters were checked for metal. Yes, 25 hours between checks means that a lot of hours could be flown before a fault was discovered. But, hey, it was better than never even having the ability to test! Quite a number of A90, B90 and early C90 and E90 King Airs are still operating with the detector test points inside their cowlings and no chip annunciators.
The 200, receiving its Federal Aviation Administration (FAA) certification in late 1973, was the first King Air to have the left and right engine chip detectors wiring into the annunciator panel circuitry. The annunciators were in the warning panel – the one with all the red lights. Red implies “emergency,” so they are very important lights that almost always involve some checklist steps that should be memorized.
So, what did the emergency checklist tell the pilot to do? Absolutely nothing! Why? Because the first 200 checklists had no procedure whatsoever dealing with this newly-installed annunciator! It appears the design engineers snuck an annunciator in without bothering to notify the POH writers.
Dave Simon, one of Beech Aircraft’s marketing staff on the international sales team, was the first pilot to experience the illumination of a chip light while flying one of the early 200s. He dutifully searched the checklist and, as I’ve made obvious, found nothing! The engine was running just fine, so he chose to continue the flight. Yes, you guessed it: Within about 30 minutes the engine blew up! Dave made a successful single engine landing and the Beechcraft team quickly verified that indeed the RGB had uncoupled. Hmmm … perhaps we’d better address that annunciator in the POH!
A POH and checklist revision was quickly forthcoming. In fact, all of the King Air models then being produced – C90, E90, A100 and 200 – started being equipped with the annunciators and their POHs/checklists had the procedure added. And what was the procedure? “If conditions permit, shut down the engine.” In fact, some models merely added “Chip Detector Illumination” to the title of the already existing checklist “Engine Fire in Flight.” Pull the condition lever and shut that sucker down!
As Mr. Simon’s incident (and now some others) showed, there could be no abnormal engine indications whatsoever making themselves known before the engine turned into a bomb. “If conditions permit” has never been clearly defined. The consensus seems to be that shutting down an engine because of a chip light would not be wise if the other engine had already been shut down for some other reason! Likewise, perhaps waiting to either land or execute a successful two-engine missed approach might be the wise course of action if the light illuminated while inside the FAF while executing an ILS or LPV approach with the weather hovering right at minimums.
I have two personal experiences to tell you about. One of my clients with whom I conducted recurrent training had the light illuminate on a sunny day while departing Rock Springs, Wyoming, on a flight back to Portland, Oregon. The annunciator appeared just as they were leaving the Rock Springs’ airspace. The 200 they were flying was used as a corporate shuttle and most seats were filled. Being in such good conditions, the crew went ahead and secured the engine. As they returned to Rock Springs for an uneventful single-engine landing, they hoped that the light had been triggered by some metallic “lint” instead of a significant piece of metal. This mysterious lint has indeed been the cause of some chip lights. In these cases, usually the main oil filter is inspected and if nothing significant is found, the aircraft is authorized to fly 10 hours more before another check. If things are still copacetic on the follow-up check, then no further action is required. (One theory is that this lint may be manufacturing residue that was not thoroughly flushed away after the engine was manufactured or overhauled.)
Well, the crew’s hopes of “nothing major” were dashed when, in their words, “We could do chin-ups on the prop blade!” The gearbox had frozen solid with contamination.
A year or two after this incident, I was conducting recurrent training for the pilots of an early C90 based at Gillespie Field (KSEE,), near San Diego, California. We flew east to avoid the busy San Diego area, and while doing air work near Thermal, California, the chip detector on the left side came on. The weather was perfect, we were light with only about half fuel and three people (all pilots) on board, so we went ahead and shut the engine down. Being close to Gillespie, we decided to return to that airport where they had their own mechanic available. As we flew, having declared an emergency with ATC, I was pleased to see that occasionally the three-blade prop would turn slightly as we changed speed or configuration. “At least the gearbox isn’t frozen, unlike the Rock Springs case” I thought to myself. “Probably gonna be a little lint.”
In the hangar, the mechanic quickly pulled the forward cowling to gain access to the detector. He positioned a metal pan to catch the oil and removed the detector. As the oil fell into the pan, I swear we could hear metal hitting metal! Maybe not, but that oil was thick with metal contaminants. Have you heard the line about “Pieces being big enough to have part numbers on them?” That’s about what we had here! I’m sure glad we shut it down expeditiously!
By the way, both this engine and its partner on the other side were very close to the suggested 3,600-hour TBO, so both were sent out for overhauls.
The F90 model made its appearance in 1978. Like the other models being produced then, it also had the chip detector on the red, warning annunciator panel. As time passed, it became obvious that some chip light illuminations were indeed a precursor or indicator of major engine damage whereas other cases simply were lint-related.
At some point a meeting of Beechcraft engineers, pilots and lawyers was held and the issue was discussed in-depth. I was not in attendance, but I am guessing it may have gone something like this: “Should we really treat this basically equivalent to an engine fire, even when there may be no immediate problem? Aren’t we exposing our company to possible legal action if a pilot were to botch a single-engine approach and landing, when in fact the airplane didn’t need to be single-engine at all?”
Based on this type of concern, decisions were made to (1) change the annunciator in forthcoming models from red warning to yellow caution, and (2) insert a checklist step demanding/suggesting the pilot not shut down the engine unless abnormal engine instrument readings were observed.
Having the annunciator red in some models and yellow in others has been a head-scratcher for many pilots and training-providers for a long, long time. Realize that there is no difference in the installation whatsoever except for where the light is placed and what color it is. If your flight department has, say a straight 200 and a 350, the light will be a warning in the 200 and a caution in the 350. Therefore, should you actually react differently depending on which airplane you are flying that day?
We each must answer that question for ourselves. Based on what I have presented here, I imagine you can guess my personal position. Yes, so long as “conditions permit,” I will be pulling the condition lever and feathering the prop quite rapidly. Now, granted, I have lots of single-engine time in King Airs because of the thousands of hours spent instructing in them. Since your level of one-engine-inoperative experience may be significantly less, I can appreciate your possible reluctance to do an immediate shutdown. That’s understandable and just fine. After all, there is a reasonable chance that the illumination does not indicate that a catastrophe is imminent. And if the engine does indeed blow up? Well, that’s a time to call the insurance agent.
I’ll wrap this up by giving you, as Paul Harvey used to say, “The Rest of the Story” concerning the scenario at Rock Springs. When the engine was sent in for repair, the shop reported this interesting bit of news: They said they had never seen an engine with so much destruction in the power section and yet the compressor section was fine. The engine had been shut down before the contaminated oil had clogged the main oil filter enough to cause its bypass to open. Hence, the No. 1 and No. 2 bearings – the ones that support the aft and forward ends of the N1 or Ng gas generator shaft – came through unscathed. Food for thought, eh?