We have all been to some sort of training class as a pilot. In these classes we have been shown the limitation section of the pilot operating handbook (POH), otherwise known as the aircraft flight manual (AFM). After the books have been closed and class is over, it is very easy for us to never think about the limitation section again. We have red lines on all the instruments, which will keep us out of trouble, right? Maybe, maybe not.
Many PT6 engines installed in King Airs have a cruise climb ITT limit that is lower than the max cruise limitation. Is this limit depicted with a red line on the ITT gauge? No, it is not. If you are flying a King Air with this limit, it should be memorized so that during climb the engine power will be set without breaking a limitation. If you have modified your King Air with different engines, you will need to look at your AFM supplement, which supersedes the original AFM limitation section, to see if the limitation has been removed or changed.
PT6 engines are very good engines. If maintained correctly, the chances of having an abnormal engine start are very low. Many clients I have instructed have never had an abnormal engine start while operating their King Air – a testament to the PT6 engines.
Let’s say we add fuel during an engine start in our PT6 engine and the ITT shoots up rapidly. What do we do next? Following the King Air 90 and 200 series AFMs, we would move the condition lever to cutoff and motor the engine for the remainder of the starter duty time. In the 300 series airplanes, the AFM says to motor the engine until the ITT gets below 400 degrees.
Why the difference? To answer this question, we have to discuss the starter duty cycle. In the 90 and 200 series King Airs, the starter is limited to 40 seconds on, 60 seconds off, 40 seconds on, 60 seconds off, 40 seconds on, 30 minutes off. The reason for the 40 seconds on is to prevent the starter from overheating. This limit is in place to protect the starter while the starter is doing all the work driving the engine (the engine is not producing any power). If you forget to turn off the starter during engine start, you won’t be the only one who has done this – the starter will just go along for the ride, not hurting anything at all. You will figure this out when you can’t get the generator to turn on. Oh yeah, I need to turn off the starter before the generator control unit will allow the starter to be a generator.
Back to the hot start. In the 90 and 200 series airplanes, the procedure directs you to motor the engine for the remainder of the starter duty time of 40 seconds to reduce the ITT. That 40-second duty time cycle should be more than enough time to reduce the ITT down below 300 to 400 degrees ITT.
Hmmm, how do I know how much time has passed since I turned on the starter? I know, I can count the time in my head during start. In reality, it is difficult to perform an abnormal procedure while keeping track of time in your head. One way to help yourself is to start a timer when the start switch is moved to the on position. If there is a hot start, when you motor the engine, you will only need to reference the timer to know when to discontinue motoring at 40 seconds.
In the 300 series King Airs, the starter duty cycle is 30 seconds on, 5 minutes off, 30 seconds on, 5 minutes off, 30 seconds on, 30 minutes off. During the start sequence, it may take 10 to 15 seconds before we even add fuel. By the time we recognize the hot start and chop the fuel, it may be as long as 20 seconds. Remember, in the 300 series airplanes, it tells us to motor the engine until the ITT gets below 400 degrees. Motoring for only 10 seconds may not be enough time to reduce the ITT.
I think the hot start procedure was written in such a way to prioritize the ITT, instead of the 30-second starter limit. This prioritization will cause the ITT of the engine to be decreased, thus saving the engine and sacrificing the starter. The engine is much more expensive when compared to a starter.
What happens if we add fuel during engine start and there is no ITT? How long will you wait before you do something? The first part of the procedure says: “If no ITT rise is observed within 10 seconds after moving the condition lever to LOW IDLE…” this does not mean you have to wait 10 seconds. It means you should not add fuel for more than 10 seconds without a light off. The more time that passes, the more fuel pools in the engine. For example, during start when you add fuel, there is a malfunction with the ignition system causing it not to spark, then suddenly it operates correctly … click, boom! All the fuel ignites. This is NOT good for your expensive engine! What to learn: If it does not light off within the normal time for your engines, cut the fuel and turn off the starter. Get the checklist out and follow it.
These start malfunction checklists do not include bold or memory items as published. They are not practical to perform as a “read and do” checklist. Therefore, these items should be committed to memory. In addition, include a timer at the beginning of your engine start sequence. It will potentially save the owner of the aircraft a great deal of money by complying with these procedures in the event of a start malfunction.
Maneuvering speed & turbulent air penetration speed
Some other limitations that are important:
- maneuvering speed (Va)
- turbulent air penetration speed (Vb)
Maneuvering speed is defined as the maximum speed at which full control deflections can be made without risking structural damage.
Flying at or below Va means that the airplane will stall before the structure is damaged by excessive loads. If you encounter a gust that causes a sudden, significant increase in load factor while flying above Va, the aircraft could experience structural failure.
Well, that sounds easy: To prevent structural damage, I will fly at Va and I will have no risk for structural damage, right? Not so much in the real world. Turbulence will increase the load factor but can and usually does increase the airspeed momentarily. Therefore, if you are flying at exactly Va thinking you are safe and you encounter turbulence, your load factor and airspeed will increase, causing the possibility of structural damage.
Another important thing to understand is that Va changes with the aircraft weight. Va decreases as weight decreases, and it increases as aircraft weight increases. It is a mistake to assume that as long as you are at or below Va, you can move the controls from stop to stop repeatedly without damaging the aircraft.
To clarify this point, 14 CFR part 25 states: “Flying at or below the design maneuvering speed does not allow a pilot to make multiple large control inputs in one airplane axis or single full control inputs in more than one airplane axis at a time without endangering the airplane’s structure.” Although GA aircraft are certificated under 14 CFR part 23, this point is still valid.
Due to this, the best practice would be to slow to the turbulent air penetration speed. In the King Air B200, Va is 181 knots and Vb is 170 knots. Notice Vb is slower than Va, giving you a buffer.
These are just a few ways in which limitations affect our everyday flying and why our training events highlight the limitation section of the AFM/POH. Often, we go to training to check a box for our insurance company, but understanding our airplane better can lead to a safer, more efficient pilot.