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ASK THE EXPERT
The History of King Air Chip Detector Annunciator Lights
by Tom Clements
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
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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
AUGUST 2024