Use of manual/continuous ignition
 

Hello.

I fly a PT6-equipped aircraft and in our AOM it says that manual ignition should be used in turbulence, heavy precipitation, contaminated runway or icing conditions.

However, some captains use manual ignition when encountering light to moderate turbulence at cruise and even descent speeds and I fail to see the logic behind that, since the engine has lots of ram air to use and there is no change in power by means of power lever.

By "turbulence", I'd take it like flying at low speeds, high AOA or when power lever movement is required, i.e. takeoff, climb, approach and landing. Other than that, I don't see how a moderate turbulence encounter at cruise speed could possibly cause a flame-out. Am I missing something?

Thanks in advance

Ignition.
 

Escape Path, remember that when a turbine engine is started the ignition is turned off. The flame continues to burn on it's own provided it is not disturbed. It doesn't cost anything to turn the ignition on, so for peace of mind, use it. It's a lot more of a problem trying to do an "air start" than to put the switch to on.
On most big jets you have to be below 28,000 ft and above 280 Kts to get a relight. Hope this helps.

Thermostat
 

Thanks for the reply. But what about the igniters' lifespan? Isn't it reduced in any significant extent by switching the ignition on rather constantly?

Well, yes, using the ignitors does wear them out, and they are a "consumable" item that cost a chunk of cash every time they are replaced, so using them when you didn't need to is a waste of money.

On the other hand, a big "bump" from turbulence can knock the fire out, so if you think there might be one coming, it's good practice to switch the ignitors on. By far the most wasteful error is to switch them on as you penetrate a bit of bumpiness, come out the other side saying "that was not so bad", and then FORGET to switch them off again. For an hour or two :uhoh:

I would suggest leaving the ignition in AUTO on the PT-6 in normal flight and using "ON" only in [actual, not perceived] moderate turbulence (or greater) and during takeoff/landing below 1500 feet. Using AUTO will make it kick it in automatically when Tq falls below a certain level (600 ft lbs I believe). The ignition on the PT6s differs from larger engines in one big way. Larger engines typically have 2 ignition systems, a high energy and a low energy system, the PT6 only has one system, and for all intents and purposes it should be treated as a high energy system, which means only use it if you have to.

As a side note, many pilots reduce power to idle routinely on that engine, it's good practice to leave it just high enough to prevent the ignition from running, the additional power is negligible and it helps extend the life of the igniters, while giving you a good degree of protection.

I think that the only possible correct answer to this question is "Do what the AFM says you should do."

The aircraft manufacturer is responsible for integrating all of the different components (engines, avionics, fuel system parts, brake system parts, light bulbs, engine nacelles, etc.) into a finished and complete aircraft. The aircraft manufacturer is also responsible for testing the finished product (the whole aircraft, as opposed to the individual components) and making sure that it operates safely and satisfactorily during all phases of the approved regime of flight.

So, when it comes to ignitors, different aircraft manufacturers may publish different instructions for ignitor operation. The procedures, limitations, and requirements for ignitor operation on a Twin Otter or a PC-6 (both of which are very slow moving, low altitude utility aircraft) may well be very different than the procedures, limitations, and requirements for ignitor operation on a King Air, PC-12, or Piaggio Avanti, aircraft that operate in a totally different environment.

The question is philosophically similar to asking "what kind of fuel can I burn in my aircraft?" If you look only at the engine manufacturer's specifications, you will find that as far as they are concerned, you can burn just about anything you want to, including AVGAS, automotive diesel, etc. But, if you look at the individual aircraft manufacturer's limitations and requirements for fuel, these will almost always be much more restrictive than what the engine manufacturer provides. This is because the aircraft manufacturer must ensure that the permitted fuel is compatible with all the fuel tank components, the filters, the fuel pumps, the allowable range of temperatures and atmospheric pressures, etc.

So, in short: Do what the AFM tells you to do, and appreciate that the instructions may be different from one aircraft to another.

FYI, below is an image of the instructions published in the AFM (Revision 53) for the Series 300 Twin Otter.

Michael

http://i979.photobucket.com/albums/a...rOperation.jpg

Is it common in jet or turbo-prop aircraft for manufacturers to recommend putting on "Auto-Ignition" or "Continuous Ignition" (or whatever a particular manufacturer calls it) for turbulence? And why? Is this an overly cautious practice recommended by some manufacturers? Or are some turbine engines actually prone to a flameout due to turbulence? What is the overall likelihood that turbulence can actually cause a turbine engine to flame out? How exactly would turbulence cause the fire to go out? I started wondering about this because I remember reading somewhere that the King Air 300 series recommends Auto-Ignition ON (i.e. "armed") in turbulence; but the King Air 200's do not mention Auto-Ignition Armed for turbulence ....... and I was wondering why the discrepancy. Which got me to wondering about it for other turbine engines and aircraft........

Better to have it ON when you don't need it than OFF when you do need it.

I really never flew the PT6 engine. But one day I was sitting in an odd crosswind in the runup area...(MU2 J)...and my left engine flamed out. Ignition not on, not reccommended for takeoff or anything except start.

But it just quit. Plenty of fuel...the only thing I could think of was an odd interruption of air flow to the engine.

Started, took off and no problems ever again.


Believe me when I say this...there are things we fully don't understand...an odd negative G in turbulence might interrupt the fuel flow for a fraction of a second...and sometimes in tough turbulence, you might have to throttle back...

I flew for many years in the mountainous west of the USA...and moderte turbulence one second could easily get worse the next.

I have also seen people forget andleave the ignition on for the whole flight...well that's just dumb.

I flew the JT8D engine on theDC9 and the 737...for dozens of years we didn't use the ignition on landing on the DC9...then one day we were told to.

go figure.

Saw the same thing with Boeing. With the 737-200 and I think the early 737 Classics, the FCTM called for continuous ignition on immediately before the take off roll and off during the after take off checks. The JT8D engine manual cautioned against using the ignition when it was not needed because of ignition life; inferring that just when you really want in flight ignition it might not operate. Then you had the wimps that would turn on the ignition in slight turbulence as a twitch factor often see in some personalities.

Because different pilots have differing views on what constitutes severe turbulence, extreme turbulence, moderate and normal turbulence then one sees the majority being frightened in moderate turbulence (ignition NOT needed) and hurriedly turning the ignition on. Again - the twitch factor varies between pilots with many new first officers scared fartless in mild turbulence and reaching for the ignition switches.

In those days for the JT8D, the Boeing advice for engine anti-icing was to turn on the ignition prior to switching on the engine anti-icing switches then turn off the ignition a few seconds later when engine parameters were stabilised. In other words continuous ignition simply was not operationally needed with engine anti-ice on.

Then a significant policy change came out of Boeing a few years ago where the 737 checklist requirement was to turn the ignition on after engine start then off after take off. Then if engine anti-ice needed in flight turn on the ignition first and leave the ignition on at any time the engine anti-ice was on. So it makes one wonder about the previous caution about the igniters having a finite life. Clearly that is not an issue anymore?

Hmmm....we did tng & circuits for 3 pilots an a then brandnew CJ2 and had the ignitors replaced at IIRC at 150 hrs. Did cost 650 USD a piece, 4 off em installed. After that, no tasinig on the aircraft they were not replaced until aircraft was sold at 1200hrs...

PT6s are totally different from the Garretts in the MU, and are in my opinion only in danger of flaming out at very unusual attitudes and low power settings.

My money is on V1......oops answer.

What on considers to be light or mod or severe trubulece OTOH varies wildly...

By personal set of mind, I've always thought it might have something to do with the sudden change/loss/interruption of air flow going through the intake. By that definition, I only put the ignition on in very strong turbulence, not just any little shudder.

One situation I also like to have the ignition on is when I'm landing in possible gusts or some sort of windshear. Why? Aircraft has low forward speed so lower quantity of air going into the engine and I've always thought that in the case I need a large increase in power to correct for speed or in the worst case, a go around, the low forward speed plus the now bigger demand of air from the engine and a large angle of attack in the case of the go around, if a shear or a gust attacks, sufficient air may not reach the engine and cause a flame out. This has only been a personal thought and is not an excerpt from any source, so usual caveats apply

GE rep stated in a briefing session earlier this week that excessive use of igniters through selecting cont ign on routinely had halved the life of the igniters for one engine type.

He also made the point that it's not just a commercial issue. Igniter deterioration also affected the functionality of the igniters, so they may not perform optimally when you really need them (say for a relight attempt in marginal conditions) if you've "used them up" needlessly.

So true of the many systems on an aircraft

That's why fight manuals on an aircraft are provided with this in mind.

Manual/ Continuous ignition is a useful thing, and using it for evry approach and takeoff regarless as well as mild turbulence is a total waste.
I have seen it years ago in Nigeria, the (South African) company's SOP's had it at transition altitude with the approach checklist...and it was ON, not AS REQD.
I had complained about it with many other pilots, but fell into deaf ears...until they were unable to start EITHER engine in a remote stand in Lagos... A/C AOC because of spark plugs! We had a great laugh! :ugh::ugh::ugh:

Yesterday, a younger colleague of mine asked if using continuous ignition in turbulence really prevented flameouts.

The engine in question is a CF6.

I offered my opinion that it is indeed cheap insurance at higher power settings found in cruise and climb, but it's probably more important at lower power settings during descent especially when accompanied by heavy rain like weather found around convective activity.

I tried to explain how the "flame" might be "snuffed" out, not due to lack of oxygen, but the flame itself being "knocked off" the atomized fuel.

It was at this point I reached the limits of my physical explanation and decided to do a search here for more answers.

This seems to be the BEST thread (after a search of the forums) that addresses my question: What exactly is the PHYSICAL/SCIENTIFIC explanation of the flameout during heavy turbulence?

Any physicists out there?
Thanks.

Which CF6 are you speaking of (and on what aircraft)? If you're talking the CF6-80C2 FADEC on a 767 or 747-400, auto-relight is basic.
Continuous ignition at high power doesn't do anything - above a certain burner pressure the electrical resistance of the air is such that the igniter can't spark and the electrical energy simply dissipates through the ignition components (which is really abusive to the system). The latest FADEC s/w on the CF6-80C2 767 and 747 installations actually inhibits ignition if the burner pressure is above a threshold.


Most newer installations (e.g. 777, 787) no longer even have a continuous ignition switch - auto-relight has been certified to be equivalent.

Thanks,

Interesting point about burner pressures and electrical resistance and resultant wear. A very good point many (including myself) don't consider. But as I said, I'm really MORE concerned about the actual physics of the flameout due to turbulence.

The engine is indeed the CF6-80C (and -80A) on a 767.

As I dig deeper, I see the practice of using ignition during turbulence as kind of "old school".

But just for the sake of discussion (assuming no auto-relight on a generic jet engine) how does the fire "go out" during heavy turbulence? We can even talk about the old "straight pipe" turbojets rather than high bypass fans.

Hi zerozero,
During heavy turbulence, if you experience -ve g, then there can be a momentary interruption to the fuel flow with a chance that the "fire can go out".

Please see:
------ OAVIAO.COM ------> hence one method doesn't suit all.

Thanks for the link. I guess I always assumed the fuel flow would be constant during the turbulence. But that is perfectly simple to understand.