"Reduced Idle Thrust". serial switch or "tuning"?

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cupra

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Unread post09 Mar 2019, 11:10

I got a part of F-16 that has a hole marked as "Reduced Idle Thrust" and ARM and OFF. Anyone knows on which planes this switch was used and how it is working?
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basher54321

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Unread post09 Mar 2019, 22:15

The RIT switch is in a 1984 F-16A Block 1 to 15 manual but states it is aft of the throttle and "deactivated".
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weasel1962

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Unread post12 Mar 2019, 01:40

Think F-16s probably used landing gear handle + throttle for idle thrust.
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MVSGas

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Unread post12 Mar 2019, 19:08

weasel1962 wrote:Think F-16s probably used landing gear handle + throttle for idle thrust.

AFAIK, that is only a PW thing. I have not found anywhere that specifies the F110-GE-100 (or -129) nozzle being affected by landing gear handle position. AFAIK, reduced idle thrust is related to older PW engine (-200 or early 220)
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sprstdlyscottsmn

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Unread post12 Mar 2019, 19:21

That's logical as the RIT was due to the lightweight early blocks skidding across the ramp while at Idle. By the time the GE and -229 motors were in play the empty weight had crept up to where that may not have been an issue.
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MVSGas

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Unread post12 Mar 2019, 19:27

Has the F-16A/B models weight increase that much with updates? I think is was more related to the old PW computer like the EEC and the hydro-mechanical control BUC.
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jbgator

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Unread post12 Mar 2019, 20:03

When I first flew the F-16 in December 1982 (Blk5/10) the RIT switch was already deactivated and the F100-PW200 did "Idle Area Reset" when the gear was down and the throttle at idle. The nozzle would drive to fully open in those conditions. This sufficiently reduced the idle thrust on the ground although a clean jet still required frequent application of brakes during extended taxi to preclude excessive speed. The GE motor I flew later in the BLK30 did not do idle area reset but the engine would open the nozzle after extended operation at idle regardless of gear position and hence would reduce thrust during ground operations. But, again, frequent application of brakes during taxi especially when light weight. I do not know much about the RIT function as a result so I don't know exactly how it worked or how long it was in operation. I suspect it was not long before PW resolved the issue with idle area reset. It may have only been on pre-production AC. Interesting thing is we found the switch housing was still present in later blocks and this was used for the chaff/flare paddle switch mounted in later Counter Measures System (CMS) application by ANG and AFRES.
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sprstdlyscottsmn

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Unread post12 Mar 2019, 20:19

Thanks for the first-hand account!
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f119doctor

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Unread post13 Mar 2019, 00:31

This topic finally got me to register and stop lurking in the forum

Reduced Idle Thrust (RIT) was a special capability of the F100-PW-200 that was requested by at least one of the European Participating nations (I believe Norway) early in the development of the F-16, who were concerned that the normal ground idle thrust would slide the F16 tires on icy runways.

As was noted earlier, when the landing gear handle is in the down position, the F100 control system schedules the exhaust nozzle open as the throttle is retarded from mid-range to idle (Idle Area Reset). With the landing gear handle up, the nozzle remains at approximately 10% open in flight at idle. Flight idle thrust is about 1200 lbs, with ground idle at 900 lbs.

When the Reduced Idle Thrust switch was activated, there was a special setting in the UFC / EEC control system that would reduce the Idle speed of the engine, lowering ground idle thrust to around 600 lbs. This low idle setting came with several complications: 1. Initial engine acceleration from this low RPM setting was very slow, unsuitable for a landing go-around. 2. The compressor bleed pressure was too low to properly keep engine oil inside the bearing compartment. These were solved with a special soleniod operated detent that prevented the throttle from being retarded all the way to Idle while in-flight (weight off wheels) and a special ejector on the engine gearbox to pull breather pressure below ambient to develop the pressure differential needed to seal the bearing compartments with the low RIT compressor pressures.

By the way, RIT would not be available with the Back Up Control (BUC) activated. The Exhaust nozzle would be closed, and Idle thrust would be 1200 - 1400 lbs.

As it turns out, the icy runway issue was not a problem and RIT was not needed. The switch was disabled and the engine breather ejector hardware was removed from the configuration early on
P&W FSR (retired) - TF30 / F100 /F119 /F135
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basher54321

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Unread post13 Mar 2019, 00:44

Nice thanks - so were you at P&W at the time?
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f119doctor

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Unread post13 Mar 2019, 01:34

39 years with P&W
P&W FSR (retired) - TF30 / F100 /F119 /F135
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weasel1962

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Unread post13 Mar 2019, 02:04

Deserves a +1 with Thanks for the share.
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johnwill

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Unread post13 Mar 2019, 06:18

sprstdlyscottsmn wrote:That's logical as the RIT was due to the lightweight early blocks skidding across the ramp while at Idle. By the time the GE and -229 motors were in play the empty weight had crept up to where that may not have been an issue.


You're close, but not quite on. The original RIT was needed to reduce brake wear and heating while taxiing. Excess idle thrust might possibly cause skidding on ice, but not otherwise. There was a skidding problem during early flight test, but that was from selecting afterburner before brake release at takeoff.
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Unread post13 Mar 2019, 13:52

I must have gotten old things I read about jumbled. Thanks for straightening things out.
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