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Acceleration Bleed Valve Question


TalonIIVito
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2% Gage error.....

92.5% equals 94.5%.....

A normal indication could be 92.5% indicated ABV\'s would be closed...below that the SSV blocks the 14th stage air to the backside of the ABV\'s and also \"vents\" the back side to allow the higher pressure compressor air to shift the valves open.

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Natops1, you\'re almost right but the gauge error could go the other way too ....

an exerpt from my post on HH

.... I have never seen an engine below 95% at FI, and on those occasions a quick check has revealed FI blade angle to be in excess of 17.5° and/or LPS to be in excess of 25.5°. LPS gives itself away normally by being in excess of 5500\"lb when coming down from x-over .....

....As someone posted correctly, the speed valve does open at a significantly lower rpm on downshift than it closes on acceleration - generally 92 - 94% on upshift and 85 - 88% on downshift. I have on several occasions had the misfortune of having a speed valve close the bleed valves during starting, TIT quicly rises to start limiting TIT (810°C) at +-35 - 40% and stagnate.....

Bottom line - if an engine is below 94% at GI or FI there\'s a good chance something is not set correctly, check RPM gauges, fuel control, coordinator and prop rigging and blade angles, and do a good performance run.

http://p076.ezboard.com/RPM-Limitations/fc130herculesheadquartersfrm3.showMessage?topicID=746.topic for the full discussion

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The actual RPM limits (using an accurate tachometer) is 94.4 to 98.3. The compressor bleed valves should never open.

The allowable error in your indication system is plus or minus 1%. The writers of your flight manual allow you guys to see 92.5% as long as the compressor bleeds don\'t open.

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I have found a phototachometer to be very handy for checking RPM gauges.

Once a gauge is 1% off, I replace it.

Same thing goes with TIT gauges. When I see a 6-degree error, it comes out. The problem there is that our TD system test sets are only as good as the calibration folks make it.

The hard-sell for me is to get folks to calibrate torque indicators. I find that an inaccurate torque gauge will make for lots of wasted effort, but some people don\'t want to calibrate them correctly, if at all.

While errors may be acceptable to the crew, it is a very bad thing for maintainers.

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  • 2 weeks later...

Allowable error on a precision tach? That is funny, I did an FCF on a plane a couple months back and the precision tach I had to work with had a limited calibration sticker on it saying it was accurate +/- 2%...

Guess I had an (in)accurate tach, lol. Ended up using a Grey box for the FCF runs.

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ALL measuring devices are subject to some error - that\'s why they\'re calibrated. You can virtually eliminate errors in certain ranges, but rarely across all ranges - I\'m sure the accutachs are calibrated for 100% and are the most accurate there, but to say they\'re perfect seems, well, ludicrous.

FWIW, I\'ve worked in avionics & done both calibration & verification testing for precision measuring equipment...

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The test cell uses an A2D converter for the speed/RPM display which needs to be calibrated every 6 months. The procedure calls for 3 values 7Hz, 35Hz and 70Hz which displays as 10%, 50% and 100% respectivly. Thus at 100% the tach generator is putting out a signal of 70Hz. However, because the wires from the tach gen travel along various looms and harnesses, and is also connected to the synchrophase, it is extremely likely that no matter how accurately the guages are calibrated, there is going to be an element of error induced. When in doubt of the rpm guage, take a quick look at the frequency meter corresponding ......

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  • 14 years later...
  • 2 weeks later...
On 9/9/2022 at 12:01 AM, Mandala said:

Any answer can explain of this question? Because flight idle limitation had change from 94.5%-100.5 % to 92.5-100. 5 %..why? Any explanation about that refer on TO? Thanks

NATOPS essentially already answered this question above. The analog indicator is allowed to be 2% low and still be considered safe for flight. There is a different RPM limitation for a digital indicator. If the RPM actually dropped below 94%, the bigger problem would be high TIT, low torque, and likely flameout. If you are reading 92.5 but the temperature and torque nearly match the other engines, the RPM is above the Speed Sensitive Valve transition of 94%.

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