Back to Back Battery Starts PT6

[x15]

Removed - Posted initially asking about Back to back battery starts. Got some good info. Company requested that I take down my posts. Thanks for all the insight guys!!!

[iflyforpie]

I don't have a ton of experience with the King Air, (maintenance only), but doing a back to back battery start has never been normal procedure anywhere I have worked.

For sure doing a cross-gen start is harder on the current limiter (some wear vs none), but regulators are cheap compared to hot sections and if I had a choice, I'd rather lose a generator than an engine.

If they are that concerned about the current limiters, they should be doing cart starts and not battery starts.

[KAG]

King Air 90,100 or 200? With the 90/100, you charge the battery between starts, with the 200 you can do that or a gen assisted start – Start right engine, once stable gen on, Charge the battery, gen off, engage engine other starter, after a certain point you turn the running engine gen on, makes for much cooler starts. I forget the numbers for these just don’t leave the gen on and try to start the other engine as you will fry the current limiter. Don’t do Gen assisted start for 90/100, just a quick charge in between starts.
I flew King Airs for 6 years and 3 companies and I never did back to back batt starts. Your company is doing it wrong.

[Conquest Driver]

Quick and dirty answer here. Some of the guys with more recent PT-6 time will likely do better.

You're totally on the right track. Any time you can get a GPU start, do it. Just make sure the GPU amperage isn't set too high or you'll take out a starter.

There's a difference between a generator assisted start (good) and a cross generator start (bad). On the generator assisted, you begin with both generators off and the operating engine wound up to whatever the flight manual says (around 70% NG). Do a normal battery start on the second engine, wait for the engine to stabilize, then turn on the other generator. This avoids the huge current draw of starting the second engine from 0%. You need to monitor the first engine to prevent is from going hot from the additional current draw, (which is why you increased the NG).

Run this by your management if you wish, and see what they say. Don't initiate any changes without management approval.

[thecaptain]

During starts we often see IIT's get to or exceed for a short period the 800 degree mark

Not sure on the 200 if the numbers are different, but usually you're good up to 1000 for starting only limitations with other PT6s that I've had experience with (PC12, 1900D, 208B) If you're looking for cooler starts, it's best to wait for a stabilized Ng above 14% (not just above, but stabilized) prior to introducing fuel. If you're always using the battery on starts I could imagine some weak starts and MAYBE a tendency to introduce fuel early, which can have some toasty results.

In some instances we also have large amounts of flame coming out of the exhaust stacks.

Cold engines... unburnt fuel being exhausted before ignition. Is there a particular series of events that leads up to that, such as an aborted start?? Does it last for long or is it just kinda like a quick shot of flame out the stack?

[Hedley]

Fascinating. I don't fly anything with PT6's but how I start the
C421B (similar to any piston twin):

- Left engine starts first - closest to battery in left wing root
- Left alternator on. Wait for battery voltage to come up
- Start right engine, with left alternator on to help the battery.

I understand there is some maintenance school of thought that it
is bad to use the running engine's alternator? Why is this?

Heck, many times when I have started the Pitts, I have seen
the ammeter go to full deflection after a difficult hot start.
Frankly, babying the alternator is the least of my worries
compared to burning the airplane down.

Do PT6's have weak alternators? Troublesome voltage
regulators? A schematic would go a long way here ...

The turbine stuff I fly has an onboard APU for the start,
so it's pretty hard to screw that up! All I need is 22VDC
and the onboard APU does the rest.

[KAG]

No it's the starter/generators (same unit, there is no separate alternator) require a lot of amperage on initial start up to spin the engine. It will fry the current limiter that joins the LH/RH electrical system. That’s the short dirty version as I truly forget the ins and outs on the BE20.

['79K20driver]

If you can get around back to back battery starts, then do it. Infact I can't think of why an operator would be doing them in a King Air or anything on wheels other that to save a little time in not charging the battery between starts. A twin otter in the bush (meaning no GPU) on floats with blade latches engaged is another matter. Back to back battery starts are necessary. Sure you could charge in between by taking #2 off the latches and increasing the power and selecting the gen on but then you're defeating the purpose of the latches. Most of the bush twin otters I flew had no problems with back to back battery starts but the ones I flew on the coast (whose batteries where not cycled very often due to constant GPU starting) had poor battery start performance and charging between was generally necessary. The docks where generally secure and blade latch engagement wasn't necessay (thus feathered props) so increasing power and charging between starts was easy and the sensible thing to do.

[x15]

Removed

[Caracrane]

Gen.assist start is a must. Start the 2, bring the gen. on line, put high idle, wait until amp. rate goes down to 30 then start number 1, the 1900D I fly for the winter does at least 10 to 14 starts a day with not more than 4 with GPU and we have no problem. They've been doing that for years. You got Batt.charge for a while but read checklist, no problem as long as the charge rate goes toward zero. Don t know for the 200 but for us the light comes on if charge rate exceeds 7amp for more than 6 seconds. Don' t need to put the battery off. And doing starts most of the time every 20 minutes.

[200hr Wonder]

We use to have all kinds of issues with battery starts back in my cold metro days. Your mileage my vary but here is general turbine engine rules that a capt. passed along to me. YMMV.

Engines are expensive, in fact the most expensive part of the airplane. Starting them is is too. I have been quoted somewhere in the neighborhood of $25/engine/start in overhaul costs for a PT6. What is hard on the engine is the heat, which gets high as the engine transitions through the critical speeds and accelerates to self sustaining speeds. You want to do everything you can to spend as little time as possible in that critical speed range when heat is bad. What causes long, slow, hot starts? Cold engines is one as mentioned. Weaker batteries is one as mentioned as well. These are 2 things as pilots that we can control the easiest.

As for tenting, get them on ASAP when landed and leave them in as long as possible. They don't take long to remove so when I worked in the cold SOP was to leave them on till everyone was loaded and the FO pulled them off just prior to hopping in. Took about 4min to get tents off and folded up with a little, OK a lot of practice

As for batteries, we had a policy on the Metro to charge them down and then charge them down a little more. At the end of the day if you have batteries that last half as long but your hot section costs less, what would you rather pay for? GPU best, cross gen good, batt start ok.

Someone mentioned a seaplane, they have the advantage of being in temp's above zero, as water is no longer water very long below that temp

[Caracrane]

The thing is that if for exemple for us on the 1900D it's 13% before putting fuel on, if you're first start of the day and the plane was outside for the night, you don't wait for 13 plus another 5% for God sake you have to introduce the fuel at 11-12% to get a start cause too weak to accelerate more and pout igniters ON for a few seconds when very cold (5-10 sec) when very humid at low OAT.

[tired of the ground]

There are a couple of options for your company.

1. Do nothing and keep paying for hot sections. Not particularly cost effective.

2. Do gen assisted starts. (different from cross gen starts) It's in the AFM and people all across the world are doing it every day. Pay for a few more batteries if the memory effect actually causes a problem. They're still a whole lot cheaper than a hot section.

3. Change over to lead acid batteries. They don't have a memory and the don't have that silly affliction of lighting themselves on fire and possibly melting through the wing skin or spar. Then you can charge the battery between starts if they still really don't want you to do gen assisted starts.

Heat is the real killer of turbines. There is a reason the transient torque is so high, it can temporarily handle ~35% more torque. It IS however a huge problem to have 10% more ITT because of the clearances and metallurgy inside the engine. Things expand and curl and start eating themselves.

[PanEuropean]

During starts we often see IIT's get to or exceed for a short period the 800 degree mark... Most recently it has been brought to the pilot group's attention that we have "had some very expensive hot section repairs".

Correlation does not imply causation. I am going to speculate here that the cause of the “very expensive hot section repairs” this operator is encountering is likely not due to starting temperatures, but caused by power setting procedures during climb and cruise. Bear with me while I try to explain my rationale.

I’m not familiar with the PT6A engines used in King Airs, but I am very familiar with the PT6A-27 and -34 engines used in Twin Otter aircraft. A quick review of the Pratt & Whitney maintenance manual for the PT6A-27 reveals that for starting conditions, the following temperature limits apply:

1) Up to 1,090° for two seconds, provided that the temperature drops below 980° within 2 seconds, and;

2) Up to 980°, provided that the temperature drops below 925° within 10 seconds, and;

3) Up to 925° for the entire duration of the start (nominally assumed to be 30 seconds).

Although the maintenance manual and the AFM both state that “starting temperatures over 850° should be investigated for cause”, - this simply because they are unusual and may be an indication of a failing component - the above temperature limits make clear that the starting temperature allowances for the engine are, in fact, very high, and that the engine will not be damaged provided these limits are not exceeded.

Such high limits (relative to idle, take-off, and climb and cruise limits) are granted for starting because during start, the engine components are assumed to be cold (not heat soaked), and the rotational speeds are very low. There is no risk of CT blade tip rubbing during peak starting temperatures, simply because peak starting temperatures are normally reached somewhere between about 20% and 35% Ng. The CT wheel won’t stretch at that rotational speed, especially when it was at ‘room temperature’ (so to speak) all the way through just 5 seconds earlier.

Nevertheless, there is an urban legend – or, perhaps in this case, a ‘bush legend,’ that suggests that if the starting temperatures go up into the amber or red zone of the T5 indicator during start, limits have been exceeded. This is not the case at all, because the limits marked on the T5 indicator don’t apply to starting conditions. They apply (in the case of the Twin Otter) to takeoff and MCP (bottom of the red arc) and climb/cruise (bottom of the amber arc).

A well maintained battery should give approximately 18% stabilized Ng when starting the first engine, presuming that the OAT is above freezing. If the starting procedures in the AFM are followed exactly (note that the requirement to ‘wait 5 seconds after Ng stabilizes prior to selecting fuel on’ was removed from the AFM in 1993, fuel should be selected ON as soon as Ng stabilizes), the battery will not be substantially depleted during the first start, and the second engine may be started immediately following starting the first one. Ng of the second engine being started will typically stabilize at about 1% less than that of the first engine, this due to the partially discharged state of the battery.

Presuming you have a well maintained fully charged battery, and presuming that the first engine stabilized at 16% Ng or higher when it was started, is therefore totally unnecessary to start the first engine, rev it up, and recharge the battery between starts. If you do elect to recharge between starts, consider that you are probably picking up a fair amount of sand and other debris while you are doing this, and that debris is eroding prop tips and also being ingested into the engine.

---------------------------

The phase of flight where PT6A engines are most likely to be thermally stressed is during climb. Takeoff is not a big concern, because just about everyone watches T5 carefully during takeoff (the beginning of the red arc is pretty hard to miss), and besides, takeoff only lasts for a couple of minutes at the most. Climb, though, is a totally different story.

Many pilots set a ‘fixed’ company number for climb power. In the case of the Twin Otter, this is often 45 PSI torque and 75% or 85% Np. There is a power-setting chart in the AFM that the pilot can use to compare OAT, pressure altitude, and Np, and from that determine the maximum permitted climb and cruise torque. A close inspection of the chart will show that as pressure altitude rises, the maximum allowable torque decreases. If pilots are using a ‘magic number’ that was handed down from their grandfather or dictated by the Chief Pilot for setting climb power, it is quite possible that the limits imposed by the maximum climb and cruise power setting charts are being exceeded.

Pratt & Whitney makes very clear in all of their training documentation that power on a PT6A engine should NEVER be set by advancing the power lever until a temperature limit is reached – that is exclusively a Garratt procedure - instead, it should be set by reference to the climb and cruise power charts. Again, a close inspection of these charts will reveal that the maximum allowable torque often results in an indicated T5 that is substantially below the published T5 limit for that phase of flight. Under these circumstances, is the pilot allowed to ‘goose up’ the power to the T5 limit? NO WAY, because doing so will result in a Torque and Np combination that exceeds the calculation derived from the power setting chart. That is what causes expensive hot section overhauls – it's not the “Oh My God it’s into the red” starts, because "the red" does not apply to starting conditions.

For those folks who are really curious and interested in investigating this further, try looking up the maximum allowable torque / Np combination for 1,000 feet above field elevation (the point at which climb power is normally set), then look up the maximum allowable torque / Np combination for top of climb (typically about 10,000 feet in a Twin Otter, much higher than that in a pressurized aircraft). You might be surprised to discover that although you are within power setting chart limits when you set xx torque and xx Np at bottom of climb, if you keep bumping the power levers forward to maintain that torque value as you climb, you will have exceeded the chart limits by the time you reach top of climb.

Do the above calculations – my guess is that the results might surprise you, and might also shed some light on what is causing the expensive hot section repairs. One thing I can say “for sure” – it’s not starting, because as long as the battery is in good condition (and, of course, both ignitors are working, and the fuel nozzles are in decent shape), starting is a pretty harmless activity for PT6A engines. You don’t have heat-soaked components when you start, and you don’t have high rotational speeds when you start.

Michael

[shannon]

PanEuropean,
Thanks for that informative post, it gave me some things to research.
Shannon

[Tim]

im brand new on the king air, so this thread has been very informative for me...we use GPU starts whenever possible though. great information from several posters.

[2.5milefinal]

As for tenting, get them on ASAP when landed and leave them in as long as possible. They don't take long to remove so when I worked in the cold SOP was to leave them on till everyone was loaded and the FO pulled them off just prior to hopping in. Took about 4min to get tents off and folded up with a little, OK a lot of practice

VERY good advice.
I can not believe what I have seen some operators doing this winter. Leaving engines untented in cold weather (-10c give or take) for longer than about 45 mins. Is just looking for trouble (imho). Really, how long does it take to put a tent on

[2R]

When all else fails read the instructions .
One of the many reasons ALL aircraft have to carry a POH /AFM in commercial air service is so that the crew can do what the aircraft maker has recomended.If you open the AFM for the 200 you will find the factory approved start procedure. It is different from early model 100's .If you are doing anything else ,you will have issues that those who are capable of following instructions do not have.
If your Chief pilot ,Captain or DOM disagree with the procedures in the AFM/POH have them put the procedures they want in writting, lest you get the blame for the improper start.
The AFM /POH is an approved certified document that has legal implications for those who do not use the procedures and numbers in it.
So find AFM/POH in the aircraft and read it .
You might just learn something

[foxmoth]

it must be hard operating for owners who know so little
If you are -20 to-40 degrees then you would think these people would know something.Tired of the Ground and KAG have it right.
Use gen assist starts. Get rid of the nicads. Some aircraft like B-02 the poh says x-gen is ok and it works, no problem. dont know why flames from stacks. Pilots should see very little of that maybe something to do with fcu settings or somehow too many nozles actuatedat to low rpm
yes, engines are most expensiv part. erosion of props is important but engines cost ten + as much, so look after them first. It takes 3 hrs at -30 with no wind for engine oil to hit zero degrees so the idea you taking too long to tent is absurd
what other wierd ideas do these peole have?

[foxmoth]

oh a second thougt about flames
sounds like you are doing quik turns. if so, you a lighting off a hot engine the ITT shud be below 200 before fuel is turned on

[Doc]

I just park the puppy on a hill, then let the clutch out on the roll...

[2.5milefinal]

"It takes 3 hrs at -30 with no wind for engine oil to hit zero degrees so the idea you taking too long to tent is absurd
what other wierd ideas do these peole have?"


..whatever

[co-joe]

So to clarify as KAG said, a cross gen start is one where the first engine is started, the gen is brought on line, after the battery voltage comes down to whatever normal is, you go ahead and start the second engine. Now in an early model king air, this will result in around 1000 amps being pulled across the left and right current limiters and one of them will blow at 325 A.

Late model 200's and the 350 have what's called a GCU that limits generator output and allows it to help the second start without allowing more than 325 A across.

A gen assisted start is where the first engine is started, the gen is turned on to charge the battery, then turned off. You start the second engine off the battery, and then when the N1 goes through 12% you bring the first gen back on line to help the second starter. On a -20 day you should be getting 600 degree starts on the second engine following this procedure, not 800+!!!!

Now my understanding is that the left engine should be started first. The battery is after all in the left wing and so the voltage will be higher with less wire to go through. Plus the captain can watch the engine they are starting for abnormal signs...like 5 foot flames! Some companies alternate left and right engine starts to spread out the damage.


If your company is afraid of gen assisted starts, they're not alone. I flew for a company in YYC where the SOP's contradicted themselves on this. One page said "all starts will be gen assisted" and a few chapters later is said never ever under any circumstances should you attempt one. They'd had a rash of current limiters blowing years earlier due to pilots not following proper procedures I would guess.

IMO doing back to back battery starts without at least charging the battery between starts is retarded. I can't believe you work somewhere that would do this, and I assume I am misunderstanding you on this. I think Beech says you need 20A on the battery to attempt a start and most companies have a minimum 22A SOP. On an old battery cold soaked you'd be close to this every time. What kind of N1 are you getting before you introduce fuel on the second start?

[It's the Pitts]

I fly 4 king airs(200), they all act a little differently. We also operate in cold Canada. We have Tannis installed, which is great. We tent. I'm a big fan of this because when you are away from base, you need to take care of your a/c. I will do everything in my power to make certain that we get home. Try flying a piston in cold wx and not tenting. Let me know how that goes for you?

As for the starts, operationally sometimes you have to do back to back battery starts (gravel ops). I find the best is the Gen assisted starts when a GPU is not available. Like other guys said a current limiter is alot cheaper than a Hot Section. And yes you do blow current limiters occasionally. Try a Gen assisted start. You can see the ITT very low 600 C.

As for the flames and smoke, it seams the colder the engine is the worse it is. I see some engines will have flames on starts and some don't.

I just wanted to re-iterate what some other guys said.

Pitts

[co-joe]

...
As for the starts, operationally sometimes you have to do back to back battery starts (gravel ops)....
Pitts

Totally disagree BTW. Start the engine normally at low idle with the props set full fine, and as the props come on speed immediately go into beta. Look straight down under your prop for the vortex and change blade angle accordingly. Give the FO operation of the right lever to accomplish this for the right engine while you are parked. Plus you used your cut off broom and swept the gravel away from under your props right?

The only occasion I could think of where you absolutely have to start back to back on gravel is if the wind is strong or gusty and shifted 180 degrees on you while you were parked but even that's a stretch. Just opinion. You gotta follow SOP's, if they say start back to back, follow em I guess.