Ask the Expert: Rambling Replies

Ask the Expert: Rambling Replies

Ask the Expert: Rambling Replies



When readers send a question to me or Kim Blonigen, our editor, I try to respond directly without delay. Some questions concern a specific item that affects so few operators that publishing the reply here in my monthly column would merely be wasting ink. However, some questions lend themselves to a wider audience. This month I am publishing some of those questions and my responses in the hope that they will be of interest to most of our King Air owners and operators.

Starting Procedure for Newer versus Older 200s

This question comes from Xandi Newell in Charlotte, North Carolina:

We have a question for Tom Clements regarding the King Air B200’s electrical system. We operate two King Air B200s: serial numbers BB-1246 and BB-1613.

On serial numbers prior to BB-1444, the generator is turned off prior to cross-starting the second engine to prevent blowing a current limiter. This requirement is not necessary for BB-1444 and after. There is much debate among our pilot group as to why the difference exists, with some speculating that the GCU prevents the operating generator from producing excessive current during generator-assisted cross-starts, but we cannot find any documentation that states this directly.

Could Mr. Clements shed some light on this for us?

This was my reply:

Yes, just as you theorized, the GCU limits the output of the operating generator whenever the opposite start switch is activated. This current limiting began with the 300-series, but was added to the 200-series at BB-1444.

With a department such as yours that operates 200s that fall both before and after the change date, I suggest that the earlier procedure – generator-assisted start, not a generator cross-start – be used as SOP for all the 200s you fly. Leaving the generator on increases the chance of experiencing a current limiter failure during the cross-start on the earlier airplanes, whereas doing the Off-On procedure does not harm the later aircraft in any way. In fact, I think it marginally reduces starter-generator drive spline and brush wear, as well as reduces the ITT rise slightly on the operating engine. By standardizing on the earlier procedure, it eliminates the “Which one am I in today?” thought process.

One additional comment, as discussed in my book: If the stabilized N1 speed of the first engine, on the battery, exceeds 16 percent – i.e., “Meets your expectations” – then you are wasting time and effort to charge the battery between starts. It’s certainly not incorrect to do so, but rather is unnecessary. So get to High Idle on the first engine, turn its start switch off, activate the other start switch, then turn the first generator switch on (Reset – Pause – On) as the second N1 gets past 12 percent or so. It saves time and can be done for all models of the 200-series. Only if the first stabilized spool-up is less than 16 percent do you need to do battery charging between the starts.

Thanks for the good question and never hesitate to ask if I can help with other questions.

My reply sparked a follow-up question:

Good morning to you, Tom!

Thank you very much for the quick response regarding the GCU on starting the different King Air 200s. It was such a delight hearing directly from you and I really appreciate the time you took delving into the differences.

I do like the notion of keeping things consistent when operating the two different serial numbers. So far, (where’s that wood upon which to knock?) I have been able to keep the two separate by always verbalizing the elements of the starts as I go along, but we all know there will be that one time… Since it’s not hurting anything, I’ll adopt that standardized procedure right away. Thanks!

I have a copy of your book, so I do indeed recall your mention of charging the battery between starts being unnecessary if we see spool-up upwards of 16 percent. What I do wonder, however, is why we wouldn’t need to introduce fuel on the second engine prior to turning on the generator of the first engine. You write: “So get to High Idle on the first engine, turn its start switch off, activate the other start switch, then turn the first generator switch on (Reset – Pause – On) as the second N1 gets past 12 percent or so.” Our checklist for the BB-1246 King Air B200 has the step of bringing condition lever on second engine to Low Idle when we get past the 12 percent, and then turning on first engine generator.

I noticed this in both your book and a video I saw on the King Air Academy’s YouTube channel, so I was curious about that.

Cheers! Xandi

My reply:

Hello again, Xandi. On a scale of one to 10, what I am about to write is probably a two or three, so not critical at all. Doing it either way is fine. But …

I choose to get as much N1 – and hence as much airflow – through the engine before I ever introduce fuel. By doing so, I guarantee the coolest possible start because of the extra cooling air. You will notice the Beechcraft checklist says that on the first start you can bring in fuel at 12 percent N1 or above. I disagree and think it should state “when the N1 stabilizes,” to get more airflow and hence a cooler ITT peak. Also, this lets you have a better feel for battery health: Does the N1 stabilize at 14 percent or 18 percent?

As long as the first engine’s generator is off when the starter switch for the second engine is activated, the chance of blowing a current limiter is basically zero. The huge peak amperage flow only occurs when the engine is stopped; no rotation. When rotation begins, the amperage demand drops off rapidly and significantly. So, by getting the first generator on now and allowing it to spin the starter faster – we see maybe 20 percent or even more when N1 stabilizes versus the approximate 16 percent with battery alone – we achieve a much cooler starting ITT peak.

So, as I said, either method works fine, but my goal is to keep temps as low as practicable and my technique does that.

More 200 (C-12) Questions

C-12 Instructor Pilot Galen Collins, of the Navigator Development Group, in Dothan, Alabama, asked the following four questions. My responses follow each question:

Thank you for your receptiveness to answering questions about King Air equipment. I have a few questions below if you have the time. I am flying US Army C-12s (King Air B200C).

As part of the engine oil cooling system, there is a vernatherm valve that will automatically open and close a small door below the engine oil cooler. The mechanism to operate the door would appear to be based on the temperature of the oil itself as I have physically seen the door in action without the engine operating. Our operating manuals do not discuss this feature and I was wondering if you might have any further information about it – how does it operate and at what temperatures makes it work?

All aircraft oil systems contain the vernatherm valve, the pur­pose of which is to control oil temperature by directing oil to either flow through the passages of the oil cooler or to bypass around them. During a typical start, for example, the oil cooler bypasses until oil temperature starts coming up. In the development stages of the 200, it was found that the location of the oil cooler – in conjunction with the then new design of the cowling – did not permit sufficient oil cooling under hotter conditions due to insufficient airflow. So, the oil cooler door was added to permit increased air flow across the cooler’s fins. You are correct, the oil temperature itself works a mechanism that overcomes the spring that is tending to hold the door closed. You have probably observed that the door may be wide open at the start of a lunch break, but then is closed an hour or two later. (When it’s closed, you can pull it open with your fingers.)

I wish I understood and could present the exact working of the mechanism better, but I do not have that detailed knowledge. All I can state is that once the vernatherm is directing all oil through the cooler – the bypass going closed – now the next step is to start opening the door.

We have various models of the C-12, but all are with the PT6A-42 engines. Aircraft equipped with the three-bladed prop system use TGT for the engine temperature gauge and the aircraft equipped with four bladed props use ITT on their gauges. ITT versus TGT – why are these different when they both get their temperature sensing at the same position in the engine?

If I recall correctly, the very first C-12 models did indeed mark the temperature gauge as TGT (Turbine Gas Temperature). It was “funny” in that those models that first appeared in 1976 were supposed to be “off the shelf” standard 200s, yet they incorporated many, many, changes that the folks at Ft. Rucker desired and this was one of them. As time passed, the later C-12 models became much more standardized with the civilian ones and the old familiar ITT came back into use. Exactly when that happened, I do not know. My guess would be that four-blades versus three blades has nothing directly to do with the TGT/ITT debate, but rather merely shows that most of the later models were built with standard four-blade props. If a previous model were upgraded to four-blades, I speculate that TGT would remain.

In reference to the autofeather system, why was 90 percent N1 chosen as the setting for when the system becomes armed versus any other setting?

The whole purpose of those left and right power lever switches is to allow the autofeather system to differentiate between a desired and commanded power reduction – as should happen when the power lever is retarded – versus an un-commanded power reduction that happens when the engine rolls back even while the power lever has not been retarded.

N1 and power go hand-in-hand, but is not even close to a linear relationship. At sea level, 90 per­cent N1 typically is closer to 50 to 60 percent power, not 90 percent power. By using this value, it almost guarantees that there will not be “nuisance” activations of feathering. Expressed another way, when less than 60 percent power is being carried, almost always flight conditions would not be as challenging as when very high power was required, i.e., takeoff and go-around.

I should probably add that if a significantly higher N1 switch setting were specified, then it increases the chance that autofeather would not arm during a low-altitude, cold-day takeoff. The values that I quoted above are ones applicable to sea level where 100 percent of rated engine power can be achieved. As you know, N1 is going up every time we push the power levers forward in the climb. So, as odd as it first seems, one of the lower N1s we will observe when high power is requested is the one at takeoff, not the one in cruise flight at altitude. If the switches were set for, say 95 percent, there are lots of low altitude, winter takeoffs in which autofeather would not arm.

In reference to the starter, when it is engaged and the N1 accelerates during the start sequence, is the starter physically turning so long as the start switch is engaged, or at some point does it become freewheeling, or maybe just “along for the ride?” I’m not aware if there is a clutch feature inside the starter. It seems that the higher speed of the N1 would at some point drive the starter. The starter is located on the accessory gearbox. Does it direct-drive the compressor or is there a series of gears involved?

Realize that the starter is also the generator. There is no clutch and it never disengages from the accessory gearbox (AGB) unless the mechanic removes the nuts on the studs! Typically, the engine becomes self-sustaining – where the energy from the exhaust gases causing rotation of the compressor turbine is enough to run the engine, without starter motor assistance – in the low 40 percent N1 range, so after that it can be said that the starter is indeed “going along for the ride” even though it is still connected. One never wants to be in a hurry to turn the starter switch off since doing so before self-sustaining speed is attained will result in N1 rollback and a hot start, if fuel is not immediately cutoff. The starter time limits do not apply once exhaust flow starts. The only problem with leaving the start switch on is, of course, that the generator circuit is prevented from operating! The POH says to turn the starter off at “50 percent or above,” but I encourage people to wait until full low or high idle stabilization has occurred. As you said, there comes a time that the starter is not doing the driving but is being driven by the AGB … when that happens, it is setting the stage for it becoming the generator.

I thank Xandi and Galen for these interesting questions and encourage other readers to send questions my way whenever I may provide some clarification or guidance.

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