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G1000 torque gauge question.

Old 16th Jul 2014, 13:45
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G1000 torque gauge question.

Up until this year, I haven't had much experience flying glass cockpit aircraft. I'm now flying G1000 equiped C208B EX caravans.

The other day I dragged one up to 18,000 to clear some terrain and weather and watched the limit on the torque drop lower and lower as we got higher.
Up until now, I have done, and witnessed other captains across numerous different types of turbines with steam gauges continually bump the power up as the torque drops off with altitude until they hit either the NG limit or the ITT limit.
But on the climb up, I noticed that the torque limit was reducing as we climbed to a point where I had to reduce power to around 97% to stay under the torque limit. Temp was also not a factor.

Why is this?

I've learnt by monkey see, monkey do. I don't want to continue this bad habit if I have been operating turbines outside of their limits!
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Old 16th Jul 2014, 14:10
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Well,

If you are refering to the Blue bug on the torque indicator it is only a recommended setting for cruise performance. And it definetly decreases as altitude increases. You should look at your POH to get climb and cruise performance charts and notice that the higher you climb and cruise, the lower the torque setting allowed.

Max Cruise at ISA and 1900rpm: (for a 675 SHP engine)
12000 Ft 1563 ft-lbs
14000 Ft 1469 ft-lbs
16000 Ft 1371 ft-lbs
18000 FT 1277 ft-lbs

Same goes for climb performance
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Old 16th Jul 2014, 14:46
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I assume you're talking about the cyan "bug" on the torque gauge... That is the "cruise torque bug" (as per Garmin G1000 pilots guide for Cessna Caravan...

It "represents the recommended cruise torque settings (appears for certain combinations of propeller speed, outside air temperature, airspeed, and altitude)"

That isn't a "limit" as such... And isn't really applicable to the climb as far as I'm aware...

For climb... You set whatever you want without exceeding 765C (or your companies SOP limit. We use 740 or 101.6% Ng
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Old 16th Jul 2014, 14:51
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No, I was actually referring to the Red line. That also decreased with altitude.

Been a bit of a long day so I may be missing something here, but why does the torque limit reduce with altitude?

Although the engine is turning at the same speed (Ng), due to the air being less dense, it is producing less torque, and therefore SHP. I thought that there would be less "strain" on the engine for the same amount of work being done.
Is it something to do with air pressure across the engine?

Maybe I'm looking at it from the wrong angle and I'm more than happy to be re-educated.
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Old 16th Jul 2014, 15:06
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Does the Caravan EX have a 'Max Torque for Climb chart' like the older 'vans???

It may be the max permissible regardless of ITT
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Old 16th Jul 2014, 15:24
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It most certainly does have one in the PoH.
Its just a shade over 1400lbs Max tq for the climb at -4 and 18,000ft

Normal van max tq at that altitude and temp would be 1180lbs.

I'm actually pretty embarrassed that evidently I have been over-torquing steam gauge vans. I'm obviously not alone as one of the other van drivers has pointed out a few post before me.

I would love to hear the reason for the the drop in torque limit over altitude! Thanks!
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Old 16th Jul 2014, 23:06
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Another example of quality training on that big cessna 172!

This really frustrates me as it's not hard to train people correctly and give them the tools to do a good job!

The moving red line is called the "dynamic red line" this is the maximum tq for take off up until 16000ft and then max climb power above that. The blue bug is Maximum cruise tq as per the cruise tq table in the POH. This cannot be relied on and needs to be confirmed by the PIC using the chart in the POH.

Max climb Tq is much much lower than take off tq and should be determined using the chart in the POH. Check out max tq for take off at 32'c sea level and then max climb tq at 32'c sea level and you will see what I mean.

The pt6-140 is not as forgiving as the pt6-114 and operators are going to have major problems if they do not follow these limits.

OCTA
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Old 16th Jul 2014, 23:38
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At no point was I referring to the blue bug. I totally understand what the blue bug is.

I am asking about the red line.

It's interesting that I you have touched on quality training. Two different countries and a sim course at Pan Am and climb torque was never touched on.

I've done a little more reading and thankfully I never hit the tq climb limits on any of the past machines due to not flying high enough for it to become a factor. Not many people take a caravan up to 18,000 I guess, hence the need to never talk about it. And my time in king airs... the things were so old and full of holes that it would could never fly higher than 18,000 anyway.

I've asked a few other PT6 drivers across different machines and they all say the same thing. Keep bumping the powers up as the tq reduces and staying below ITT or Ng limits. All of them fly steam gauge aircraft.

I've obviously learned from this mistake, but now rather than just continue with monkey read, monkey do.... I would like to know what causes the tq limit to reduce with altitude?

Last edited by lilflyboy262...2; 16th Jul 2014 at 23:52.
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Old 17th Jul 2014, 00:05
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For climb... You set whatever you want without exceeding 765C (or your companies SOP limit. We use 740 or 101.6% Ng
Once upon a time this was true but at a point of time a "MAXIMUM ENGINE TORQUE FOR CLIMB" chart was introduced to the POH which appears to be overlooked by a lot of operators, check departments, instructors/trainers and C208 pilots themselves. Although this change happened many years ago what has unfortunately occurred is "monkey see, monkey do" in regards to teaching flying of the van (i.e. "this was how I was taught so this is how I will teach it to others") and few have actually consulted the books or kept pace with procedure changes. It also hasn't been helped by the fact that other PT6 aircraft (e.g. Kingair) don't have a similar climb torque limit (not justifying pilots not keeping up with changes though).

I no longer have a current copy of the C208B handbook as I don't fly them any more so this page reference may be a little out of date, but the above mentioned chart is found in Section 5 Page 18 of the C208B G1000 POH of the no longer current copy I still have (not sure on the page number for the non-G1000 version but the same chart is in section 5 somewhere). The limitations section for the engine in the POH also directs you to this chart.

lilflyboy262...2,
As for giving you an answer as to why this limit exists I am sorry (and disappointed) that I cannot answer that for you because I don't know myself (and would like to know). My former company had a "PT6 expert" (from an engine overhaul firm) come give us a lesson on PT6 design and operation but when a colleague of my asked him about the climb torque limit and why had it been introduced he didn't know about it, couldn't give us an answer as to why it was there, said he would contact P & W to find out and we never heard back from him.
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Old 17th Jul 2014, 00:08
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The climb tq limit is something that has only been introduced in the last 8 years and it seems a lot of old courses still don't teach anything about it and some instructors/check and trainers still choose to ignore it.

This is my educated guess but purely that as I am open to be corrected. The climb tq limit is there to keep you under max itt and Ng. This is the same reason as Takeoff Tq limits all they are there for is to give the pilot a Tq setting that will keep your ITT within limits.

This makes it simpler for a pilot to have a figure to set instead of taking a best guess.

But as I say I stand to be corrected!

I am impressed you are actually reading the book and looking for answers there are too many people who just don't bother these days!
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Old 17th Jul 2014, 00:36
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I suspect OCTA has the idea.
On the climb torque chart it does state that the values obtained will remain within limits.
To support that theory if you look at the time & dist to climb chart it fails to mention the climb trq chart and only the limits.

Torque set at 1865 foot-pounds or lesser value must not exceed maximum climb ITT of 765*C or NG of 101.6%
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Old 17th Jul 2014, 01:51
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If the limit is there just to ensure you dont exceed Ng or ITT then can lilflyboy assume no real harm has been done if he hasnt actually exceeded those limits? Its nothing to do with the gearbox?
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Old 17th Jul 2014, 02:29
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I'm not endorsed on the Van, nor do I have experience with that particular PT6 (plenty with the -67 and -42).

However.... I agree with Blatant. It sounds like Cessna put that mark on the guage so that Joe Blogg's with 200hrs could fly a Van. Keep it simple maybe.

If the ITT, normal engine torque and NG haven't been exceeded, then have they really had an engine exceedance by going past that 'climb torque'?

morno
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Old 17th Jul 2014, 03:35
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So, I just happen to have a copy of the 208B EX POM infront of me, I was also intrigued by this question and did some research but couldn't find any particular reason for this limit after a bit of googling around but did notice the following Note under Maximum Engine Torque for Climb it says:
1. Torque on this chart shall be achieved without exceeding 825c ITT or 103.7 percent Ng

Where as for the "Cruise Maximum Torque" it states:
1. The Highest Torque shown for each temperature and RPM corresponds to maximum allowable cruise power. Do not exceed this torque, 805c ITT, or 103.7% Ng, whichever occurs first

As the Maximum Engine Torque for Climb doesn't mention the line "Do not exceed this torque" unlike the Cruise Maximum Torque section and does mention the ITT and Percent Ng not to exceed I'd say that OCTA and others are correct and it's merely a simple way to ensure you don't exceed your ITT or Ng limits with less fiddling around than simply setting whatever gives you just under the limits.
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Old 17th Jul 2014, 05:09
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Hey morno……. you turbine guys keep saying how wonderfully easy they are compared to pistons.

As someone who is reasonably qualified to comment on the piston ops……MYTH BUSTED buddy
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Old 17th Jul 2014, 07:35
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Is this another one of those lawyer written PoH things where someone in the States cooked an engine by staying at max continuous tq but ignored the ITT or Ng limits?

I wish I had taken a pic of the engine parameters and not just the altitude I was at! The tq limit had moved down to the book value of just a shade over 1400. Yet the Ng and ITT were at 98% and roughly 780*C. Well below either of those limits.
It seems extremely an conservative number if its just a figure there for dummies. What is so hard about setting the engine power to remain under whichever of the three needles hits its limit first?
The problem with this dynamic line is that if it is just a precautionary line, that you then have to ignore it, get a flashing red tq warning, and use the standby tq meter if you needed the power to continue to climb.

It really is baffling me that an engine at 8,000ft in the cruise can happily make 1800ft/lbs until it runs out of fuel but is only allowed to produce 1400ft/lbs at 18,000ft (ITT and Ng considered).
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Old 17th Jul 2014, 08:40
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Take this with a grain of salt, but...

I believe the climb torque limits came about due to Garrett drivers setting a torque for climb without reference to itt or ng (apparently they're less forgiving), so Garrett introduced these limits so all you needed to do was look at your altitude and oat, then blindly set torque and be "certain" everything else is within limits. Cessna then adopted this practice. So, climbing generally, if your itt and ng are within limits, and your torque is below max for that rpm (208b 1865ftlbs @1900rpm) it is impossible to overtorque the engine.

Now for the g1000, if you sit stationary on a runway and apply power until the prop governs at max rpm, then pull the pitch back, you will see the red torque line increase. It's purpose is the same as the torque charts, if you're lazy and dont monitor the other parameters, as long as you dont go over the red line you should be ok. It protects your engine based on current config and conditions.

If I'm off the mark (very possible) please feel free to correct me, it's how I learn.

Cheers,
Sox
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Old 17th Jul 2014, 11:08
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The climb tq limit is something that has only been introduced in the last 8 years and it seems a lot of old courses still don't teach anything about it and some instructors/check and trainers still choose to ignore it.
Yes, seems that way. I've flown the first generation vans quite a bit, and never recall coming across this limitation in the original 80's operating handbook in the aircraft.

I've just spent some time looking through a 2004 POH and cannot find a reference to climb torque limitation there either, just Ng and ITT.

Did something suddenly change in the engine in the last 8 years?
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Old 17th Jul 2014, 11:09
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As far as I am aware, in your example, the pulling the pitch back and watching the red line increase is a gearbox limit.

SHP is worked out by Tq X RPM divided by 5252. So in reducing the RPM, you can increase the torque and produce the same SHP.
Hence the two red lines on the steam gauge vans and that little table that is printed on the panel in front of you.
I think the first three figures all give the same amount of HP (within 1 or 2hp).

I'm getting the funny feeling that this is one of those "Never operate lean of peak" PoH things.....

Maybe I will take this over to the tech forum and see if I have any joy there!
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Old 17th Jul 2014, 12:56
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SHP is worked out by Tq X RPM divided by 5252. So in reducing the RPM, you can increase the torque and produce the same SHP.
This always made me wonder. Say you left the RPM at the max setting and then increased the throttle to the maximum torque figure for the lowest RPM setting, other than having the engine output more than its book rated SHP what is the harm being done? No particular component is working harder* than it otherwise might have?

*or should I say beyond its limits.
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