B777/747 reverse thrust

[EGKK.]

hi all,

can anyone tell me how much fuel either/both of these aircraft typically use on landing using full reverse thrust compared to idle reverse?

Also which current aircarft on the market may use the most?

thanks.

[Rainboe]

Strange question, and almost impossible to answer apart from 'a few kilograms'. So please say what your level of knowledge is and why this is important. Full reverse not usually used- only possible for a few seconds anyway, most airlines usually just use idle reverse. So why is this important?

[EGKK.]

It's simply for my own interest. with fuel costs as high as they are and airlines and their employees trying to do their part in conservation of it' i'd be interested to know how much can be saved in using idle reverse rather than full reverse on landing on these two types of aircraft.

i specifically chose these two types as larger 2 and 4 engined aircraft on the current market, as i assumed they would tend to possibly use more fuel than other, smaller types. ie A320.

Also, i fail to see how this question is odd in anyway as this is a furom for discussions on this matter isn't it???

And how my level of knowledge comes into it i fail to see why this would matter...simple question, how much fuel would be used, theoretically...

so if anyone can be kind enough to answer my question the best they can i'd appriciate it. :ok:

[Rainboe]

Now I can answer better knowing from what basis you ask! Any saving in using reverse idle is counterbalanced by increased brake wear. I believe most airlines find the most cost effective method is to use reverse idle only- that way reverse is available in seconds if really needed. I believe the Qantas 747 accident at BKK was made worse by not habitually selecting even reverse idle. Few shorthaul airlines use more than idle reverse- noise is a big factor too. Fuel counters only click down in 10s of kgs. It is also available only from nose down to 6o kts anyway when reverse idle must be selected, so not for long. So I don't believe a pilot can answer as the only difference is a clicking down faster for a few seconds, but nobody is looking at fuel gauges at that stage. Reverse power is not 'efficient' reverse- it makes lots of noise for relatively little effect.

[EGKK.]

ok, thanks for the info.

i undertsand it would be almost impossible to gain a figure from looking at the gauges at the landing stage as you've already mentioned that they're not that accurate nor can you determine the exact fuel figure at the instant you touch down visually but there must be some figures out there some where.

i'm sure boeing know the answer but i was hopeing someone here might know a rough figure...this is based on good landing conditions. light wind, dry runway, good LDA. how much can i expect to save approx?

[enicalyth]

30 secs of taxi time?? Easily masked by the slings and arrows of outrageous fortune

[Screwballs]

With the caveat that this is all more or less on the back of a very small cigarette packet...

Using the Boeing QRH and some basic maths I achieved a rough figure of 21.3 kilograms burned if you touchdown at a groundspeed of 145 knots, using 74% reverse (detent 1) and autobrake 3 to slowdown. That gives you just shy of twenty seconds to get to 60 knots where you would normally go back to idle reverse. All based on a 737NG.

If you use idle reverse, with the reverse adder, to get to the same 60 knot point you would burn circa 8.2 kilograms.

So a difference of 13.1 kilograms. Very rough maths but I would say I can't be out massively! So in my end of the business, 13.1 X 1000 sectors per day = 13.1 tonnes of Jet A-1.

Jet Fuel Price Monitor (http://www.iata.org/whatwedo/economics/fuel_monitor/index.htm)

Using the above that could be a saving of $17,218 or £8,648 in real money per day.

All of the above based on a pilot's grasp of mathematics and the venerable Boeing 737-700. :ok:

[oz in dxb]

EK is now asking crews to consider using only idle reverse if situations allow.
My very basic calculations would be something like this.

At max reverse I assumed a fuel flow of around 3000 - 4000kgs/hour.
Using 4T/hr = 4000kgs/60secs. Average reverse time about 20secs which would equate to 22kgs per engine, so around 40kgs for each landing. Not a lot but as Screwballs calculated over a period of time the costs add up.

Figures for the B777.

Oz

[EGKK.]

That's what i'm talking about!

thanks boys, good stuff...:ok:

[SNS3Guppy]

I suppose the emphasis is the most important consideration. Is saving a few ounces of fuel the critical issue, or something else? For us, we use reverse because our braked landings in autobrakes reduce the braking in proportion tothe amount of reverse. That is, land with greater reverse, land with less wear and tear on the brakes, and cooler landing gear. That means shorter turn around times, and there's really no time around the clock when the airplane isn't getting turned to go back out again. It never sits, and the limiting factor as often as not is brake energy and temperature.

Accordingly, it's more economically sensible to use the reverse for us, and as low a brake setting as possible, given our weight, the runway length, and ambient conditions.

[PEI_3721]

Fuel saved? Probably not as much as required by the tug to pull the unfortunate who ends up in the mud off the end of the runway.
Rising fuel cost - a threat to safe operation which requires careful, balanced judgment in order to stay in business. Money or mud; either can fail an operation.

[Rainboe]

What does that mean? Don't use reverse and you will end up off the end? For the last 20 years, it has been practice on my operation 747 and 737 to use idle reverse as standard. I can't recall any run-offs

[PEI_3721]

Rainboe, “… does it mean … you will go off the end”, no it does not, nor was this stated.
It is an example/reminder that safety risks might be overlooked or unanticipated during times of change. Those who always use reverse and might contemplate using idle instead could face greater risks.
Also, a reminder of the need to balance commercial and safety interests at a time when commercial aspects are in the news, until the next overrun.

[SoundBarrier]

EGKK I see that you're looking at fuel conservation here, and the first thought that came to mind is about the fuel used during taxi, from what I recall a tad more that 23kg! :)

There must be a way to save fuel during taxi (tugs mentioned before but you gotta balance the cost of tug, tug production and tug use etc) oh and apparently a conveyor belt MAY save you space but the conveyor would use fuel to get it running too. :}

SB

[Fright Level]

Lets see, a very rough estimate. A 744 uses about 55 litres a minute taxiing, 210 litres a min with cruise power set and 420 litres a min at climb power.

Estimate the reversers run somewhere between cruise and climb power for 20 seconds, then I expect the burn to be in the region of 100 litres for the reverse power segment? Idle reverse say 50 litres and full reverse 150 litres as a rough guess?

[Bolty McBolt]

Some rough figures

Max reverse thrust on 744 RR is about 80% of takeoff thrust, therefore your reverse thrust fuel consumption
would be approx (round figures only) 80% of takeoff fuel burn

7800 kgs per hr at T.O. x 80% = 6240 kgs per hour = 104 kgs per min

20 secs of full reverse = 35 kgs of fuel for a full power reverse landing

4 engines x 35kgs = 140 kgs

All figures are from engine run memory so please excuse me if the T.O. figures are out

Regards
Bolty

[EGKK.]

so in theory you could save approx 10tons of fuel by using idle reverse after 70 landings or so on a B747.

...But i just want to question something...since when is 20seconds a sensible figure..

how many of you have been using max reverse thrust for 20secs after touching down, it seems more like half that.

[mutt]

IATA fuel conservation group advised our management that the airline would save US$1,000,000 p.a. by adopting an "idle reverse" policy.

Mutt

[bigduke6]

from GE Flight Operations Newsletter, Vol 1, Issue 1, Fall, 2006

"An additional means to save fuel during flight operations is
optimizing the use of reverse thrust when operating
conditions allow. Again, it is important to consider all FCOM,
performance, airline and safety procedures in the evaluation
of the amount of reverse thrust to use. Obviously the benefit
of reduced fuel burn and reduced FOD ingestion associated
with lower than maximum reverse thrust must be weighed
against operational requirements, the potential for increased
brake wear and the operational considerations associated
with the potential for increased brake temperatures. The
potential fuel savings, assuming idle reverse thrust is used on
all engines for 20 seconds, are shown below."

I can't get the table to copy, but it shows;

CF-34 39lbs (2 engines)
CFM-56 30lbs (2 engines)
CF-6-80 137lbs (2 engines)
CF-6-80 267lbs (4 engines)
GE-90 149lbs (2 engines)

as the average difference between idle and full reverse for 20 seconds....

[Rainboe]

Although you do hear aeroplanes using reverse power sometimes, they shouldn't reaally be using it. It is standard policy for noise reasons to only use idle reverse, so high reverse power is quickly available if needed (translation from idle forward to idle reverse can take several seconds before high reverse power is available). It is not only for noise reasons- also maintenance cost. Reverse power for bigger engines is difficult to engineer reliably, and very prone to breakdown. The maintenance cost is high. As a result, brakes get more wear, but changing brake units on planes is far cheaper and simpler than keeping big fan reverse systems in order.

[dany4kin]

Croydon's trams apparently use a system whereby they can store the energy absorbed by the brakes during deceleration and use this to assist in them getting underway again. This results in less power being used from the (unsightly!) overhead supply.

Could this be developed for use in aircraft for a fuelless (excluding APU) taxi out/in or would the extra weight of the gear just burn off all that 'saved' fuel in the cruise?

While on the subject of saving money, how about using the system mentioned above to spin the wheels up before touchdown and save on tire wear? Not an engineer or anything, just have strange ideas sometimes!

Got a call back from Dragon's Den about another idea but when they realised it was just an idea they weren't interested!

[Vibes]

Hi All,

Does this mean that the A380 engines 2 and 3 will succumb to more wear and tear as the reverse thrust is only available for those two engines?Sorry to veer off a lil with regards to the a/c type.

Warmest Regards,
Vibes

[lomapaseo]

While on the subject of saving money, how about using the system mentioned above to spin the wheels up before touchdown and save on tire wear? Not an engineer or anything, just have strange ideas sometimes!


Probably because it costs more to install and service the motor and control switches that spin the wheels.

I use to get these ideas a lot from my young engineers. After I sent them back to their calculators to do a cost estimate comparison, they never came back again.

It's easy to be immaginative, it's tough to be practical.

[Rainboe]

Does this mean that the A380 engines 2 and 3 will succumb to more wear and tear as the reverse thrust is only available for those two engines?Sorry to veer off a lil with regards to the a/c type.
The additional machinery for reverse on those engines will certainly take a lot of maintenance and be prone to breakdown. I believe Airbus were all for leaving reverse out altogether, but it was customer demand that led to reverse being installed in 2 engines only.

[Vc10Tail]

Reverse thrust on any operation should be conservative and idle reverse is the general recommendation for gas turbine engines be they turbo props/jets/fans.

Economics becomes redundant where safety issues of a particularly marginally safe to unsafe landing conditions or pilot excursions(correlated with experience and recency levels on the type) are factored.

If the weights calculated correlate to those loaded(i.e no fraudulent weights) the performance data should guide the pilot quite accurately(even with airframe and engine degradation-as performance engineers look into such factors).Now its up to the pilot to fly the bugs as referenced for the scheduled performance computed.Anything excessive will eat into both saftey and economics but we are dealing with infinitismaly micro economics.

Even for a stabilized and on the bugs reference speed landing It could throw all the perfection out of perspective if the airline has an urgency to meet the gate arrival time due to schedule tardiness or ill passenger and pilot tries to short cut a runway exit taxiway...hence instinctively gulping more fuel with more reverse thrust.

Reverse thrust for most turbojet/fans is more effective at high engine RPMs
(ref DP Davies-Handling the Big Jets) and so if landing weights, runway length and surface conditions,pilot experience level do not bias toward safety demands...then Reverse thrust is just a luxury(aerodynamic braking and good old wheel brakes are sufficient to do the job...that is WHY Reverse Thrust IS NOT FACTORED in scheduled landing performance for category A airplanes.

Taxi fuel allowance for a heavy jet is in the vicinity of 200kg.Fuel burn on landing is roughly only10% of that.

You can try to save pennies if you like(and they do count with a massive scale of operation no doubt!)but ask yourself the fundamental questions:

Is it at the expense of safety?...at the expense of macro economics,scheduling,maintenance cost,noise penalty fees,etc)?...at the expense of passenger comfort levels(especially an early morning landing)?...at the expense of noise rules(environmental sensitivity)...and so on and so forth....:rolleyes: