Photos by Clint Goff
Among the King Air aircraft my company manages is a King Air 350 for which our pilots fly missions and our maintenance shop turns the wrenches. It’s a wonderful situation as we have complete control of the overall management of that airplane. If a negative situation develops, it’s either our fault or our opportunity to correct. It is fantastic if everything goes well (we get to take credit for good management) and can be terrible if something goes wrong (we own that problem).
Something did go wrong recently with that King Air. As part of the phase inspection, the fuel nozzles on both engines were due for a flow check and inspection (every 400 hours). With the fuel nozzles removed, it is a perfect time to accomplish a borescope inspection to make sure everything is OK in the inner bowels of the engine. And, while you have the borescope out, it’s a good idea to look at the first-stage compressor blades with that same borescope. This is a normal maintenance rhythm at any quality King Air maintenance shop when nozzles are due.
Well, wouldn’t you know it. The right engine had FOD (foreign object damage) on two of the first-stage compressor blades. Sometimes FOD is small enough to be blended – a technician literally has a small piece of sandpaper on the tip of the finger and monotonously rubs the blade until smooth. But oftentimes the limitations for the damage require that the blade be repaired or replaced off-airplane at an engine shop. If so, the engine is removed, crated and sent to the engine shop for disassembly.
When blades only need to be blended, a sigh of relief can be heard in the next county. Blade blending is super-cheap and easy in the realm of engine troubles. Alternatively, you’ll hear a gasp of fear from two counties over when an engine must be removed from the airplane and sent to the engine shop for FOD repair. No one wants the cost to split open an engine before the next major engine event.
When a PT6 engine must be split open, it is not just the offending first-stage compressor blade that must be repaired or replaced, but anything else that is discovered out of limits in the engine must also be brought within limits. Sadly, to get to the first-stage compressor, the engine is completely exposed. It’s like going to a doctor when you have the sniffles: you’ll have the sniffles addressed, but a good doctor is going to look at the whole body on any exam. Many an early cancer diagnosis was discovered by a good doctor during a sniffles exam. Kudos to that kind of doctor. And kudos to the engine shop that looks at the whole engine when the first-stage compressor blade FOD is repaired or replaced.
The FOD event on our King Air cost $83,000. Let that sink in. An unexpected $83,000 invoice is a stomach punch for just about anyone regardless of financial position. The repair also cost the owner two months of downtime. For an airplane that operates 200+ hours each year, two months down is a lot of logistical inconvenience.
Those dang jumping pebbles
This raises the question, “What caused the FOD?” What was that foreign object and where’d it come from? In my experience, 90% of the time you’ll never know the source of a FOD event or when said event occurred.
Sometimes FOD can be determined by the shape of the damage. I’ve seen a blade that had a perfect indentation from the head of a small bolt, and it was easy to trace that damage back to poor maintenance practices. More often you’ll see strikes that look like an itty-bitty shark bit a piece of the blade; there’ll be a rounded area missing on the leading edge. Sometimes, multiple blades have multiple damage events.
Did a single pebble strike multiple blades or multiple pebbles get ingested? It’s hard to tell. The induced flow of air will ensure that the object(s) get sucked farther into the engine, causing more downstream FOD. If the first stage is damaged, there’s usually more damage to other parts of the engine until the offending pebble is crushed into small pieces by the fast-moving turbine and spat out the exhaust.
Death by a thousand cuts
Another type of FOD can be from ice in flight. Ice can collect on the leading edge of the inlet or on the prop. With ice present and then the prop heat switched to ON, the ice flies off the prop and/or inlet lip in chunks and goes into the inlet. That ice will cause the first-stage compressor to look like rough grit sandpaper. It won’t be one or two shark bites on the compressor blades, but there’ll be thousands of small indentations. It’ll be death by a thousand cuts. The compressor blades will feel rough to the touch, which degrades the aerodynamic effectiveness of the blade. Whether a pebble, blade of grass, ice or tiny bolt left by an inattentive maintenance technician, the damage will have to be repaired.
In-flight ice damage is quite easy to avoid – turn on your ice defenders early. The “hot five” (pitot heat left/right, stall warning heat and fuel vent heat left/right) should always be on in flight. I can’t think of a single reason to ever not turn on these critical defenders for the entirety of every flight.
Prop heat is another defender I recommend be turned on anytime you think you might enter an area of cold wetness. Effectively, there’s no penalty for turning on prop heat. You’ll not burn up the heating pads on the individual blades, and it’ll cost nothing more than electrons. Your generators will burn a few more amps, but the generators on any King Air have huge capacity. With prop heat selected ON, you eliminate the potential for ice to develop on the protected part of the blades. In my rookie years of King Air flying, I rarely turned on the prop heat except when I knew I was about to need it. Now, in my much more mature King Air flying, I have entire flights using prop heat.
Deflectors on defense
Pilots dislike turning on the ice deflectors. They believe it’ll slow them down, lessen their rate of climb and burn more fuel. They are right to a small degree. Turning on the ice doors will impact all performance parameters negatively, but in their haste those pilots have forgotten the huge cost of FODing out two engines. The bottom line is that in-flight ice must be avoided and is easy to avoid by simply turning on your ice defenders before entering icing conditions and leaving your defenders on until well past the time that ice is present on the airplane.
Then there’s the elephant in the ice room: proper use of the ice deflector (or inertia separator) on the ground. The King Air pilot community has diverse opinions on management of the ice deflectors, but most FOD potential exists on the ground.
This was the crux of the matter when considering the FOD event on our King Air. We have three pilots who operate the King Air 350, and no one knows which pilot had the FOD or where it happened. Human nature wants to place blame when something bad happens, but blame won’t fix the damage. The only way to serve the pilot community is for us to learn from others’ mistakes, for that is far cheaper than learning from our own mistakes.
Only one engine had FOD damage, and only two blades in that engine were damaged. So, it was not an in-flight ice FOD event. It was probably a singular pebble that caused the FOD. It likely struck one blade, bounced around the inlet a bit, struck the other blade, then was sucked deeper into the engine. A single pebble seems so innocuous, but that single pebble was the probable cause of a lot of damage. Whether in your engine or your shoe, pebbles matter.
The compressor turbine of a PT6 engine turns about 36,000 rpm when at 100%, but those speeds are only seen in the upper reaches of flight at high power. On the ground, we usually idle the Ng of our engines (depending on variant) somewhere between 52% Ng and 65% Ng range, so this means that compressor turbine is turning somewhere between 18,720 rpm to 23,400 rpm at idle. That’s an incredibly fast speed! A thin turbine blade doesn’t stand a chance spinning at such speeds when a pebble of any size hits it. The wise PT6 pilot will do anything to avoid even something so small as a pebble in the engine inlet.
Door #1 or #2: Which does the pebble choose?
So, how did that pebble enter the inlet? There are only two ways a foreign object can enter an engine: through the inlet on the front or through the back door underneath the engine. The front of the engine is protected with an ice deflector. That one ice deflector switch moves the deflector in the front as well as opening the door in the back where debris can escape.
When the ice deflector is off, the front door is up (not diverting air) and the door in the back is closed (meaning anything that goes into the inlet has nowhere to go but into the engine). If the ice deflector is on, then the front door is down (diverting air downward) and the back door is open (allowing debris to be ported overboard). With the ice door on, that pebble has a chance of jumping into the inlet and then going out the open back door quietly without any resultant damage to the engine.
The ice deflector works best when there is more airflow into the engine (induced flow), and most engineers will advise that the ice deflector really doesn’t do much deflecting until 40 KIAS, which is when there’s sufficient induced flow. Understanding induced flow is key to knowing how to defend your engines against the jumping pebbles. To get a foreign object into the engine inlet, we need two things to occur: 1) a force sufficient to lift the foreign object to the elevation of the engine inlet and 2) a flow of air (induced flow) into the engine to suck that foreign object into the engine. Where do we get the force that lifts the foreign object? From the propeller.
That problematic prop
there’ll be a rounded area missing on the leading edge.
The propeller has two main ground operations that have enough force to lift a pebble: excessive use of beta/reverse and when the propeller is feathered. The classic case of the jumping pebbles happens on landing. The pilot lands the King Air and slams the power lever into reverse, but the ice deflector is off. The prop wash on the ground is immense and blows the pebble into the air as it bounces and skips from the air being blown before the prop. With the ice deflector off, if that pebble enters the inlet, it has nowhere to go except into the engine. If that pebble gets past the squirrel cage – the affectionate name for the inlet screen – there’ll be a FOD event.
To prevent FOD events, use beta/reverse sparingly during landing. If needed on a short runway, use it. However, avoid excessive beta/reverse during normal landings. While better than rolling off the runway, beta/reverse introduces FOD risk. Ensure the ice deflector is on if using beta/reverse.
Use beta/reverse quickly after landing, as it’s more effective at higher speeds. Below 40 knots, it produces little reverse thrust and induces little flow into the engine, reducing the ice deflector’s effectiveness. One hallmark of a professional King Air pilot is that they rarely use beta/reverse regularly on normal landings.
The other area where pilots must consider the FOD potential is when feathering the propeller. The prop dumps the oil that is holding the prop blades in flat pitch, and they suddenly feather. This sudden movement sends a whoosh of air in all directions and can disturb debris under the propeller, potentially blowing pebbles from the ground and ingesting them through the open back door (if the ice deflector is on).
Insight from other markets
The King Air community has gained insight from other aircraft with PT6 installations much lower to the ground. In the Piper PA-46 market there is a PT6-converted airplane called a JetPROP. I fly these airplanes often, believing them to be super-cool airplanes. In the JetPROP market, the PT6 engine is nearly 2 feet lower than on any King Air, meaning that jumping pebble doesn’t have to jump as high to cause some real damage.
The JetPROP POH advises that the prop be feathered first but feathering the prop first can disturb any foreign objects under the prop. We advise a pilot to shut off the engine with the condition lever while the prop is still in flat pitch, wait three to five seconds (so the induced flow is greatly reduced and the prop starts to slow down), and then feather the prop. There is less induced flow, less whoosh when feathering and far less potential of a FOD event.
I have started doing this procedure in the King Air with great success. Since the prop is attached to the power turbine (not directly linked to the Ng turbine), it doesn’t care whether it is feathered or left in flat pitch on shutdown (it will eventually feather when the oil pressure drops). You will not hurt your engine or propeller by feathering (or not feathering) manually. And you won’t create a FOD risk if you delay the feathering until shortly after the engine is shut down.
If you are the owner and the pilot of your King Air, you want to avoid a FOD event because you are writing the checks and you won’t want downtime. If you are a pro pilot, you want to avoid a FOD event because non-pilot owners don’t tend to understand FOD events and they don’t appreciate unforeseen invoices. Avoiding the FOD event is altogether the best path and it is doable if the pilot understands how FOD can occur.
We’ll never know which of our pilots caused the FOD event and we don’t care. I’m certain none of them intentionally damaged the engine. While negative events can happen, they can be mitigated by aware pilots implementing good operational practices. We conducted a pilot’s call with all our pilots to discuss the points outlined in this article. I hope this information serves as a reminder of good operational practices that can keep your King Air out of the engine shop.