Flight Idle and Flight Detent

These two terms are used for the Boeing 737/747 and 777. I have looked lots of places including Boeing operating manuals but cannot find 1) a definition for Flight Detent and how and when they are used and 2) Exactly what is flight idle? Does that involved pulling the throttles all the way back, part way back, a little way back? I don't know. Typically what i have done for years when starting a descent in a Boeing as mentioned above I use between 2000 and 2500 fpm and pull the speed back to about 230 to 240. But its always been a guess on my part. Can someone enlighten me please. I'd be forever grateful but right now it doesn't look like forever is as long as it used to be. Thanks in advance.

The engines have different idle settings for the different stages of flight. IE: flight idle

will be higher N1 than ground idle.. Flight detent refers to the speed brakes.

For a B747 Flight Planning and Performance Manual

 

Descent

Time, distance and fuel information is shown based on flight idle thrust at VREF40 + 70 KIAS and straight in approach

 

Normal Configuration Landing Distances

Flaps 15

Dry Runway

Reference distance is for sea level, standard day, no wind or slope, VREF15 approach speed and two engine detent reverse thrust

I'm not sure either of those responses answer my questions but i appreciate the effort. Thanks. On the throttle itself are the words "Flight De Tent" with a line. I assume it means to pull the speed brake back to that line at a given time. What is that time?? On the Throttle, there are numbers on a scale under the throttles themselves. If you are flying at say FL32 and they tell you to lower your altitude to FL20 you would normally reduce the throttles to Flight Idle BUTTTTTTTTTTTTTTTTTT What I'm asking is what is that level? On the 737 it actually has something that says Flight Idle BUTTTTTT I don't know if the top of the throttle goes there or the bottom of the throttle goes there. What do y'all do when you start descent?

Those words are not on the throttle it is on the speedbrake lever ONLY.

There are four speedbrake settings: Down (detent), Armed, Flight Detent and Up.

 

They are two entirely different things. Flight Idle in the 737 IS a power level that is either set by the autothrottle (35% N1 with no weight on the wheels and levers pulled back) or 21% if the autothrottle is not engaged.

 

As this can make a substantial difference to float at touchdown and approach speeds the rule of thumb is autopilot off = autothrottle off. At altitude and in the Flight Levels you will not disengage the autothrottle so any Flight Level Change will be managed by the aircraft systems based on a minimum N1 of 35%.

 

The reason it is called Flight Idle is because it is not a fixed setting, it is a calculated result and depends on the systems and how they are operated.

 

Clearer?

 

Good site for describing the controls on a 737 - the 747 is pretty much the same -

http://www.b737.org.uk/flightcontrols.htm

In a jet like a 737/747/777, ground and flight idle are both set with the thrust levers at the idle position. Flight idle is a higher idle setting automatically applied by the fuel control unit, usually when approach flap is selected.

 

The flight detent is the maximum position for the speedbrake lever for in flight use. The detent position is marked on the throttle pedestal. In flight the lever can't be moved beyond the detent. Moving the lever beyond the flight detent is possible on ground and extends the ground spoilers and increases the angle of the flight spoilers.

 

For descent, assuming you don't have an FMC profile to follow, reduce thrust to idle and maintain cruise airspeed. If using a default FSX aircraft you could set A/T speed to 250 but maintain airspeed higher than that using the V/S control. Decrease airspeed below 250 knots as you reach 10,000 feet. I usually aim for 240.

 

Hope this helps,

 

Kevin

Flight Idle is often a detent position on the throttles, similar to the detent that stops the throttles from going back into the reverser position. Frequently, it is a "gate" that comes across the throttle lever throw range; a small bar on the inside of the quadrant, that is able to be overcome, or pushed out of the way, by sufficient pressure, that comes out when the WOW switches de-energize. Sometimes Flight Idle is a position on the throttle range to go past which you must move the levers to one side, lift up on, lift a small lever attached to the throttle lever it's self, or whatever.

Basically, Flight Idle is a slightly higher Idle position for the engines, say 72% N2 where-as Ground Idle is 68% N2, for example, it helps reduce the chances of sudden compressor stalls or flame-outs. You should see the neat trick you can do with the J79 during a high power run-up on the ground. If you take both engines up to MIL, cut one to idle then back to MIL real fast you DO get a compressor stall, and a big jet of flame shoots out the intake, goes up over the cockpit area, and down the other intake. If the guy in the cockpit is a boot, and doesn't close the canopy, and you can talk him into doing it, usually as a "test" on a new or repaired engine just installed, he will usually loose most of his hair and eyebrows. Hilarious, if it's not you in the cockpit :D Anyway, no flight idle stop on the ground, soooo...

I may well be wrong, but usually, I think, Turboprops have it, sometimes the big jets do, at least 3 military fighters do (the F-14D, F/A-18C/D, and the T-34, and IIRC the F-4, according to the NATOPS manuals), and very seldom do piston engines, if ever.

 

I don't know if the top of the throttle goes there or the bottom of the throttle goes there.

If it's marked on the throttle quadrant, I'd wager the bottoms of the throttle levers should align to the fixed mark. Usually the top goes up to a MAX, or MIL, power mark. On Military afterburning, or reheating for our overseas friends, aircraft, often the throttles must be pushed outboard slightly, then may continue past MIL into AB, like on most American planes with ABs. On the Israeli Kfir, the handgrip the pilot holds onto on the throttle lever (the throttle lever looks like an inverted L , horizontal bar pointing inboard) is lifted up about 30° to permit AB range.

Does all this answer you query?

Pat☺

They are two entirely different things. Flight Idle in the 737 IS a power level that is either set by the autothrottle (35% N1 with no weight on the wheels and levers pulled back) or 21% if the autothrottle is not engaged.

Flight Idle is nothing to do with Autothrottle and disengaging the A/T on aproach will not allow you to set a lower N1.

Great, thank you one and all for your great input. I appreciate it and the light is much brighter now on both subjects. Explanations very clear.

You can watch the flight idle change during the typical descent. At TOD, to

go to idle might drop to the low-mid 40's or so N1. As you descend it will slowly drop.

By the time you are under 10,000, it will usually be down around the 37-38 N1 range.

Then as you further descend, it will continue to drop. At touchdown, it's still in the

process of spooling down, and I'll usually be in the mid 30's or so N1 at touchdown even

though the idle is actually lower if you gave it time to fully wind down.

Ground idle is around 22 N1 as I recall.

WOW switches

 

Is this "modernese" for "squat switches?" Has political correctness forced this change in terminology due to some ingrained fear of using any word that has more than one context, like "squat?" :eek:

 

Even in NATOPS and aircraft maintenance manuals they were called "squat switches." Never heard them called anything else! :confused:

 

We return you now to your regularly-scheduled thread, already in progress.

IF I may quote, and I do:

 

A1-F18AC-NFM-000

W on W weight on wheels

WOW weight on wheels

WOWB weight on wheels both

PG. 49

 

 

2.10.1.3 Weight-On-Wheels (WOW) System. Numerous aircraft systems function differently

depending upon whether the aircraft is on the ground or is airborne. The most important of these

functions include: various flight control laws, landing gear operation, master arm and stores jettison,

fuel dump operation, AOA HUD indexers and approach lights, Automatic Throttle Control (ATC) and

autopilot operation, pitot static, AOA and total temperature probe heating, fuel tank pressurization,

and the inflight IDLE and afterburner lockout throttle stops. To determine when the aircraft is on the

ground, a proximity switch on each gear indicates when there is weight on each of the wheels. There

are a variety of failure conditions which may result in false indications of WOW or weight off wheels.

These include: a misrigged landing gear WOW proximity switch, a WOW proximity switch failure, an

improperly serviced landing gear strut, a landing gear control unit failure, or a problem with the

aircraft wiring.

An uncommanded pitch up after takeoff may occur if a WOW system

failure results in the aircraft sensing weight on wheels while inflight.

The first indication of the aircraft sensing WOW while inflight is the inability to raise the landing

gear handle. Other possible indications include the CHECK TRIM caution, NWS on the HUD, and the

CK FLAPS caution if the FLAP switch is moved to AUTO. The aircraft may then quickly undergo an

A1-F18AC-NFM-000

Pg. I-2-64

 

 

NAVAIR 01−F14AAD−1

12.1.7 Weight On−Off Wheels Switch Malfunction

There are WOW switches on the left and right main

gear that interact with many aircraft subsystems to provide

safety interlocks. The interlocks prevent operation of various

components or systems on deck or in flight, as appropriate.

Failure of the left or right WOW switches to the

in−flight mode can cause loss of engine ejector

air to the IDGs and hydraulic heat exchangers

causing thermal disconnect and/or heat damage

to the generators and aircraft hydraulic systems.

 

 

Shall I go on? :D ;) :rolleyes:

 

Not PC, at all. Let's face it, the '14 and '18 are gettin' a tad long in the tooth, after all...

I'm not quite as dumb as I look, thank goodness! I'd be in REAL trouble

Pat☺