Thermocouples used in big jets.... AS coursework
 

Hi there, I'm just doing my AS level coursework and am using a thermocouple, testing specs, accurace and all the rest of it. I was just wondering if anyone had any interesting uses or thermocouples on modern jet planes. The anti-ice system would be obvious I'd have thought, or in the engines- can anyone spcifically tell me how they work in particular situations and why they are used?

Champ

Not sure about anti ice use on other A/c but not on the type I work, but definately on brakes and engines.

A/I systems tend to rely more on thermistor's for temp control & overheat protection.

Gas Path temp on engines almost exclusively use T/couple's:ok:

Engine Exhaust Gas probs typically 7-11 probes all round connected in parallel to a junction and then forward with compensated leads. Probs gen alumel chromel junction. Typical fault is EGT under reading or spikes in the EGT trend.

Only ever come across Chromel & Alumel type probes used in the hot junctions on aircraft.

yes chromel and alumel are the most common,thinks its in the k-type congif,but copper and constantan may be used for measuring ''cylinder gas head'' temps as i read in mod15

so is an average taken from several different probes (I would assume so). Also, approximatly what temperature will these probes measuring engine exhaust be measuring?

Thanks

Slight difference between TGT and EGT

Early engines used EGT as it was a lower temp enviroment
Current engine use TGT

so what do TGT and EGT actulyl stand for?

thanks

Turbine Gas Temp and Exhaust Gas Temp I think.

old ones used JPT too (Jet pipe Temp)

There is also ITT on some (Inter Turbine Temp) - it just depends where the T/couples are mounted (without meting)

You actually want the temp AT the turbine but it can be inferred from cooler positions.

does anyone know spcifically where thermocouples are mounted on Trent 800s? and spcificalyl how many and spcifically how hot they get?

thanks

Sorry to be a drag- could someone tell me the main reason thermocouples are used insted of thermistors? and any other reasons


thanks

Thermistors don't tend to handle temps over about 300*C due to materials.

Thermocouples can be chosen to handle all sorts of tmp ranges due to the choice of material types.

Thermocouples generate their own voltage too so no need to run current carrying power cables into a hostile environment (even if they didn't melt)

Oh and Thermocouples are cheap and easily produced.

Apologies for minor thread drift - I've been working on engines for 20 years and am still amazed at how the EGT system for the RR engines and other marques fail in the most unpredictable manner. Does anybody else see this - or am the only unlucky powerplant wallah?

PS - TET - Turbine Entry Temperature

PPS - Remembering that the wires from the T/C have to also be alumel chromel....

Brgd's

N1 Vibes

N1,
Must say that TGT failures in my experience are not that common.

The RB21 has 17 thermocouples that all route through a ballast resisitor.
Serveral themocouples can be u/s (Open circuit) but not have a dramatic effect on readings.
I have know up to half a dozen u/s with no marked indication problem.
There is usually a few degrees drop in indicated TGT
Of course there is also indicator failure but generally these are obvious faults
Sticking or FSD.

RR Allison T56/501 engines have 18 thermocouples (3 per combustion liner). Any 1 thermocouple failing can cause a loss of up 24°C TIT at take off power. Not a lot you say - probably true, but the TD system will then increase fuel flow, i.e. increase TIT, until TIT signal from thermocouples matches input from the coordinator: result turbine overtemp, and eventual burn-out of turbine.

GTT-shirt,

wasn't suggesting RB system is unreliable - in-fact I agree that I've only seen perhaps 1 headscratching defect a year and the local airline has 500 engines on-wing. But, the point I wanted to make was the peculiarity of some of the final fixes. Especially now with causes including FADEC technology, C-duct heatsheld material interference and even one event on a CFM where the Arinc 429 data wire in the pylon actually caused an EGT signal fluctuation.....

Brgd's

N Vibes