I am an AOA driver at LAX and from time to time I will see a plane waiting for a gate. Some of the planes seem to sit and you don’t really know their engines are running. Other planes are sitting and their engines seem to be “rev’d” up quite a bit. Is there a reason for sitting with a higher or lower throttle? Also will the brakes hold the planes (think 757, 767, A350 etc…) with a full throttle? very rarely I will see a plane in the alleyway hooked to a Tug and they will throttle up quite high for 5 mins or so. Any guesses as to why? Lastly is there a “neutral” gear where you can throttle up but not move the plane?
Lots of possibilities. First there is no neutral. The engines always produce thrust and the faster the engine runs the more thrust it produces. Not sure the exact reason some airplanes might seem like they are sitting with or taxiing with a high power setting. One reasons could be mechanics are doing maintenance or checking for proper engine operation. Another reason could be the pilots are doing a cross bleed start. It requires a lot of air to start a jet engine. Usually there is a small jet engine in the tail portion of the plane called an APU (auxiliary power unit) that provides the required air. If the APU isn’t working then the other engine can provide air to start the other engine. To do this the running engine has to be throttled up quite a bit. Also some times to save fuel the pilots can either shut off an engine while taxiing in or they can choose to start only one engine when they first leave the gate. If the plane only has one engine running then the single engine will need a much higher power setting to make the jet taxi. Hope this answers some of your questions. PS. On airliners the brakes are strong enough to keep the plane from moving while it is at full power. The question is do the tires have enough traction. On dry pavement yes they do. If you start adding things like water or snow then the tires can start to slide and that’s not a good thing!
Some turbine engines have an idle time limitation for oiling, after which they need to be run up for a short period of time, but Jim's explanation is pretty complete.
I can't remember if it was the B-52 G or H but the brakes could not keep the airplane from moving with all eight engines at 100% when it was at light weight.
I will add the fighter type aircraft can usually hold with full military power, full military power is maximum thrust with out afterburner. With full afterburner fighter type aircraft have to be chained to reinforced concrete anchors.
Agreed for most. F-16 won’t sit still at mil if I recall. It’s been a while but we ran up to 80% on t/o holding brakes then pushed up and into AB at brake release. I may be forgetting, to be honest.
In most cases with fighters the limitation is the tires and not the brakes. The brakes will hold but the tires start slipping and not necessarily uniformly with resulting skitter and skipping. Our old F-111s had huge brakes and huge tires, so you could run her up to max A/B as long as the runway was dry. T/W at normal T/O weight was not as high as an F-16 or F-15, however.
Construction at LAX. I have been here for 7 years. working on the new terminals. SWA and Delta. Before that Hospitals (county and private) and LAUSD Schools.
