Why does the B738 take so much runway?

[MIAemployee]

Hi guys,

I was wondering, why does the B 737-800 takes so much for takeoff.

A boeing 757 or 767 takes less, than the B737-800.

And I was wondering why it takes so much runway ?

Thanks...

[QAVION]

I have to ask, MIA, if the 737-800's you watch, always start their takeoff run from the same point at the larger jets? (If you can't see the other end of the runway, they not be starting off from the same point. Also, which airlines frequent your local airport? Some may have a policy of "derating" their engines, to reduce engine wear and tear (Engine takeoff thrust can be reduced by programming the Flight Management System). If the runway is long and dry, there should be no problems extending the takeoff run.

Hope this helps.
Regards.
Q.

[wingtip777]

maybe we should check both performance manual of 757 and 738, then we will know if 738 really takes much than 757,right?

[MIAemployee]

Here in MIA I see the AA 738's, BWIA 738's, and Miami Air's 738, all of them take a lot of runway for departure.

I think the best reason is the "Flight Management System".

BUt is that system exclusive of the 737 NG?

Because the 752, and 763 I see take half of the way, and why shouldnt they be also limited?

Like the runways in MIA are long, thats nice, but I imagine the AA 738 taking off in Grand Cayman where the runway is really short.

Regards...

[dvt]

Two reasons come to mind immediately for the longer takeoff rolls...

1. Most takeoffs are made with flaps 1.
2. Thrust selection is made to accomadate something close to balanced field conditions. If we can safely make that takeoff with 22k vs 24k, we take advantage of MIA longer runways use less thrust and save wear and tear on the engines.

The 757/767 generally has Max thrust or standard thrust (slightly less). In either case, both these aircraft are generally takeoff with way too much power to get the job done. The 737NG "dial a thrust" system permits thoughtful selection of just the right amount to thrust to get the job done.

These I believe are the reasons for your correct observation that 737NG takeoff rolls are, in many cases, much longer than 757/767. However, at any point in the takeoff roll, the 737NG can select the full 26k thrust at any time if needed.

[Pilot Pete]

dvt

The 757/767 generally has Max thrust or standard thrust (slightly less). In either case, both these aircraft are generally takeoff with way too much power to get the job done.

The 757/767 that I fly have both fixed derates and assumed temperature derates and as a matter of course we always use the assumed temperature method unless something prohibits it's use.

Perhaps the 738 (which I am not familiar with) is operating closer to it's maximum performance most of the time?

PP

[dvt]

"Perhaps the 738 (which I am not familiar with) is operating closer to it's maximum performance most of the time?"


I'm qual'd in 737NG/757/767. I would describe the above statement as somewhat inaccurate. When calculating takeoff performance, we look for the lowest thrust selection that can accomadate a given runway length and temp. However, at anytime during the takeoff roll we can TOGA max thrust of 26K. While the paperwork from dispatch often reflects balanced field conditions, we can, at pilot option, increase the takeoff performance margin. So while the 738 may appear to operate near "max performance" it is something we do intentionally for economy reasons. There is often plenty of thrust held in reserve that's a simple TOGA click away.

When I pull the paperwork, and I see a takeoff weight spread between planned takeoff weight and max takeoff weight of less than 5000# (close to balanced feild conditions)....I will often increase my thrust selection from 22K to 24k or 24k to 26k if I don't like the dispatcher's planned weight spread.

The 757/767 in my experience doesn't operate near max performace (balanced field) very often. The is thrust available is too much IMO, and from a wear and tear perspective....wasteful. A recent article in the USA pointed out that 757 is losing popularity to the 738. And Boeing is considering shutting 757 production down as a result. This is because a 738 can do pretty much the same job for less money. The 738 is, in many ways, a much better airplane than the 757. I love flying the 757, but the 738 is a better buy for most operators.

[MIAemployee]

BUt the B738 does not offer the same range as the B752.

The B752 could get you anywhere.

[dvt]

"BUt the B738 does not offer the same range as the B752."

That's why I said "pretty much" the same job. The 757 has an obvious range advantage, but all things considered particularly for CONUS and Euro Continent operators, the 738 is a better money maker. Obviously, some markets require the legs a 752 can provide.

[Maxrev]

Now fly 'buses, but as stated above, the 757 is massively overpowered for the job. Don't care what the beancounters say - I loved it!

[Doors to Automatic]

I once experienced a shorter t/o run in a 757 than in a Cessna - about 1800ft.

I was in the jump seat on an ABZ-LHR sector and the captain told me that on this occasion we would be doing a full-power take-off.

We went off like a rocket and the initial climb rate was a staggering 6000ft per minute. Amazingly for this short sector we climbed up to FL410 - yes 410!

One of the most amazing flights I've ever done - I love the 757.

[Bomber Harris]

A very important point to this is not how much runway it uses to take off.....it's how much runway it uses to STOP.

As correctly pointed out both aircraft use the assumed temperature method to develop the minimum required power to get airborne safely. So at an airport like MIA both aircraft could probably get airborne in, say, 40% of the runway lenght. Maybe one would beat the other hands down....I don't know. But thats not important.

If you work backwards from the end of the runway and figure out where each aircraft must apply max braking at V2 (go...no go decision speed) in order to stop by the end of the runway then you will find the 738 will require less runway to stop than a 757 (it is much lighter!!). So when we derate the engines, the ideal is to derate as much as possible thereby saving the engine life as much as possible. However, the most you can derate is to the power setting which will get you to V2 at the point where you have enough runway to stop for your particular aircraft. This will change depending on temperatures, weight, airfield elevation etc etc)

Therefore, all aircraft with cost concious pilots on board are not lifting off where 'they are capable of lifting off'. They are lifting off 'at the last safe point they can lift off'. Your observations go to prove that a 738 can stop in less distance than a 757.

You will see that some aircraft will lift off before a 757 even though they can obviously stop in a shorter distance. This is because with all aircraft there is a maximum to which you can derate. This is mainly because in the even of an engine failure you must have enough power on the remaining engine to carry on with the takeoff and climb after V2. So on max derate a Ba146 will still lift off before a 757 on full power (probably!!).

I have tried to explain this without getting too technical. Sorry if it sounds patronising to non flyers...and to the flyers, I know I left 99% of my perf course out...but I only have a few paragraphs to explain!! I know I'm out on a limb with the reference to being lighter but it makes the point!!!

[mutt]

Bomber Harris,

I think that you should change V2 to V1 :):)


Mutt.

[Bomber Harris]

bugerme........:O :O

[Pilot Pete]

Not saying it won't, but will a 738 necessarily pull up quicker than a 757? Surely it's not the relative weights that dictate the stopping distance required, more to do with the retardation devices that each a/c has and their ability to stop the said a/c? Remember the 757 has 8 braked wheels. Could those 8 and rev thrust pull it up quicker than a 738s 4, let's say at MTOW in both with exactly the same conditions? Just a bit more food for thought. Any idea dvt?

PP

[MarkD]

There's quite a performance jump between RB211-535s on a 752 and CFM56-7 on a 738 (40,000lb -E4 vs 27,000lb -7B). Even with PW2037s the 752 has 36,000lb donks under each wing.

Look at the A340-200/300 which was massively undergunned with CFM56-5s (31-34k lb) - the 53,000lb Trent 500 on the 500/600 models is just in another thrust class completely.

Makes one wonder how things would have turned out if the A342/A343 had been made with RB211s instead of CFM56s... would climb performance been as derided as it seems to be at present?

[Bally Heck]

Could it be because a 757 or 767 operating shorthaul routes is highly unlikely to be anywhere near it's max structural take-off weight? They both eat up runway at MTOW for sure.

Typically a 757 on shorthaul route with a full load of passengers and bags will have a TOW (or TOM if you must) of less than 100 tonnes for a max structural TOW of 113699kg.

A 737-800 is likely to be operating much nearer structural limits and consequently nearer to field performance limits is it not?

I understand also that the 737-800 has performance issues when landing on short wet runways.

[MIAemployee]

Here in MIA I saw a CENTURION DC-10, taking all of Rwy 9R for takeoff, the crew managed to rotate at the end of the runway.

Is the DC-10 so hard for takoff?, or he just did the same thing as what a crew of a B738 do ?.

Because he almost had an incident.

Maybe he was too heavy.

[expedite_climb]

Bally heck - no not really, it is to do with the particular runway / obstactle weight limits, and most of the time the 738 and the 757 will derate its power so that it is at the runway / obs / tyre limit for each particular case.

What you say is correct if they are at full power.

Many of the 757's used as described in the UK have artificially reduced MTOM's in order to reduce overflight charges.

Perhaps it is complicated by whether or not other performance includes an increased V2 for better climb etc.

As the 757 (and I guess the 738) have less flexibility over climb thrust derate, perhaps the 738, being less over powered uses increased v2 procedure for obstacle clearance more, meaning it stays on the rwy longer ????

[G-ALAN]

I think the 738 is pretty bloody powerful. At Prestwick I've watched it get airborne in about 3000ft and it climbs like a bat out of hell compared to all the other aircraft that come and go at PIK (apart from the jaguars and hawks :D ) As a pax on the 738 we once cruised at FL360 between Stansted and Prestwick, which is only a distance of about 280nm! and it took less time on the -800 to reach FL360 than it does on a 737-200 to reach FL310. :D

[Bally Heck]

Expedite,

Because of this greatly reduced MTOW, and the fact that you cannot legally derate by more than 25%, this means that at many airfields, the 757 and 767 are taking off with considerably greater thrust than is required. I would assume that this is seldom the case for a 738?

[Sheep Guts]

Doors auto :)

once experienced a shorter t/o run in a 757 than in a Cessna - about 1800ft.

Well it may have felt like 1800ft Sir but, really?

I dont fly jets, but I am on a DHC-6-300 Twin Otter which you all may have heard. Its a STOL 19 seat Turbo Prop, still very popular in the PNG New Guinea and Africa and any other Countries that has short strips or short commutes.

Take off Distance on a 35 degree day, nil wind, nil slope, at MTOW which is 12,500 lbs, sea level, and Temp adjusted torque computer power Setting will give a take roll to 50 feet agl of the following:

1. FLAP SET 10 deg-------1750 ft

2. FLAP SET 20 deg-------1350 ft.


Now your telling me a 757 can nearly match this?

May remind all of you, when you watch these Jets take off, do you know what their respective loads are?

Because in the Otter for eg. when Im light we can get Airborne with 20 Flap in around 800-900 if needed.

Im at MBJ and I see the 738S take off here, AA and Delta and yes, they use alot of Runway in the busy period. Runway here is just over 8000ft.

Virgin Blue in Australia have 738 and 737-700. The later version they aquired with the smallest powerplant available as they were made for another Carrier that didnt go thru with the lease. These Jets pose obviously different calculations and performance than a standard -700. Its the same with - 800S it depends on the Engine thrust option ordered with the Aircraft.

Generally though the "Hair Dryer" CFM56 doesnt have the Thrust of a "Roller" OR BIG "PRATT". Hence the reason there on all the Plastic Buses and New Gen 737 which was incidently derived to compete with the Airbus hence the same Engine selection.

In the end You would have to get the books out guys to compare. Maybe someone out there can do the Math if they have the Manuals. 757, 737-800 Comparison or whatever.


Regards
Sheep

[expedite_climb]

Bally,

You are quite correct, but not sure on the 757 'most airfields' will be well below the max derated MTOW.

Dont know about the loads and fuel that MIA emp is referring to, but with a 233 pax today, LGW - AGP, 15 tonnes fuel, TOW was off the top of my head circa 94 tonnes. Well below 113400 (or as we call it 102,000). Yer the derate was to 53 degrees, the max derate being 57.

Homeward bound with just 180 pax, 13 tonnes, AGP rwy 14 (out to sea), yes it was a max derate, but only just.

I suppose the answer would be to get hold of some performance charts for miami ??

[wingtip777]

the 777s have 12 braked wheels , but i believe that 777s take much longer runway to stop than 738s with max weight! right?

[slingsby]

At least they used the runway !!!!:D :D