Take off roll
 

As a frequent flyer I'm used to the initiation of the take off roll which usually starts with a reduced thrust and then take off thrust after a few seconds once moving. Is this to reduce engine wear or to save fuel ? On the same subject, I was observing this from the airport hotel beer garden (man) a couple of weeks ago whilst waiting for a colleague's flight and it was intersting to note the PIA 747 take off. He must of held the brakes on for a good 5 seconds with the engines roaring away! Are these guy's really that short of runway or is it just the way they do it. I've seen these things flying low on more than one occasion and I live 20 miles away from the airport.

Take-Off!
 

Matblack,

What you hear while riding in the back is not reduced power, followed by max power. It is normal to 'stand up' the power levers, pause, check for stable power and the rest of the parameters, then advance to either reduced/flex power or to max power.

As for the Karachi special out of Manchester I too have seen that one and would like to see the thing run over some scales prior to take-off?? They are using MAX POWER all the way. They stand at the end to get all engines up to max power and then release the brakes. It is a max effort take off for them at the weights they are using.
It is surprising that the CAA have not shown an interest in that operation a long time ago, but then it is usually in the evening when the CAA are in bed/pub!!

what about the rolling takeoffs, on a few occasions on both 767's and 747's we have taxied onto the runway without stopping obviously lined up and shot off

Boeing procedures permit static takeoff rolls and rolling takeoffs.... so whats the big deal?


Mutt. :)

Worth also mentioning that unlike the piston engine you'll be used to on cars and light aeroplanes, jet engines have a fairly large "spool up time".

I'm not sure what it is on the big jets, but I remember it becoming very important once on some work I did on the Hawk where the time from idle to take-off power was about 8 seconds. That could at-least contribute to a perception of a delay in applying power.

G

SpeedBird48 is correct. The procedure is to set an intermediate power setting (in our case 1.10 EPR) and check for equal and symmetrical thrust before applying take off thrust.

LH

There was a good discussion on takeoff roll and spooling up some time ago. You´ll find it at:

http://www.pprune.org/forums/showthr...s&pagenumber=1

The actual timing of brake release in the whole process of spool up, stabilise, then full/flex power is quite important from a jet blast design process. Many runway and jet blast clearance designs assume that there is about 100m between the end of the runway and the achievement of full take-off power. I´ve agonised over this a couple of times where clearances were tight. It can make quite a difference in the design blast velocity.

It is not quite as haphazard as it seems, because there is some statistical element of probability included in this. If a runway end is close to the fence with a public area of road beyond, then if only 1 in 10 aircraft winds up while standing on the brakes, then the risk of a person/vehicle passing behind during start of takeoff run is reduced by one order of magnitude.

Hi,

matblack, I have also seen this at MAN. They spooled the engines to full take off power with the brakes on and wait about 20 seconds. :eek:

BAe 146-100

You wouldn't want to hold max power on a Classic for 20 seconds- it is a very good 'hoover-upper' and that is not a recommended procedure with 747s for surge reasons. If you are desperate, just clock max power and let go the brakes as soon as poss otherwise it is unnecessarily alaming for the occupants. Because of slow thrust lever response, 747s now wind up to 1.20 EPR before advancing to T/O power (which is most likely to be reduced thrust). Unless the runway is limiting or weight almost maximum, there is no need to hold on the brakes at any time- rolling takeoffs are recommended and use much less runway time for ATC reasons.

BAe 146-100 is right. They do put max power on and hold it for what seems an awfully long time. On that particular day it was the 5th 747 to t/o in the 1 hour or so that I was there. The other 4 took off with an initial roll before hitting t/o power. The PIA gave it a real smoker before letting off the brakes and inching away. By the way, BAe 146-100, i've flown on numerous occasions on your type and find it a nice safe plane to fly on. The only problem is the cabin air. It always seems less clean somehow. Is that a design limitation?

One night at Sydney, I was sitting next to the runway watching an Antanov AN-124 depart, it sat, lined up on the runway with takeoff power set for 6 mins, as timed on my watch. Why would they have so much power on for sooo long??

Wr.t. what 'overRun' wrote: If a runway end is close to the fence with a public area of road beyond, then if only 1 in 10 aircraft winds up while standing on the brakes, then the risk of a person/vehicle passing behind during start of takeoff run is reduced by one order of magnitude.

If you want a good laugh ( either that or if you're simply mad ) go to Skiathos and stand the southern end when northerly departures are in operation.

From experience there always seems to be crowds of folks, litterally just spitting distance from the aircraft ( assuming one could spit into the take off thrust of a B737 ) when it's lined-up for T/O, with the barrier between the two seemingly no more than a rusty bit of broken down wire fence.

Yes, it truly has to be seen to be believed, i.e just how close are men, women and children to the fence, and hence the aircrafts jet wash, when we're applying typically almost max t/o thrust from a standing start ( it's not a place to ****** about is Skiathos ) ! :eek:

That said, there's a lovely little bay right behind them so maybe many of'em get to take an early and maybe somewhat unexpected dip ?! ;)

On the subject of take off rolls, I have seen numerous 744's of BA take a much longer take off roll when departing LHR-SIN as compared to the corresponding SIA flight that departs around the same time. Any reason why this happens, is it just the alleged SIA fuel policy, or are there other reasons??
PS - Is the CAA going to wait till one of these PIA 747's runs out of runway before anything is done?? Seems to be a regular occurence to me!!

Maybe BA is carrying more cargo as the policy is to maximise cargo in bellyholds as opposed to SIA's dedicated freighter fleet. Maybe we carry higher loads as I think we op fewer services to SIN. As for fuel reserves, doubt if there is a great deal of difference. We also always maximise thrust reduced take-offs. Other than that I can't comment on SIA's operating practices.

This thread shows there are many ways to skin a cat. Always assuming one wants to in the first place.

Hi,

Ozgrade3, the Antonov 124 does a engine test before every take-off. That would explain why you saw it set take-off power for six minutes before the actual take-off roll.

http://www.!!!!!!!!!!!!!!/photos/small/3/8/1/290183.jpg

BAe 146-100

Guys, when you saw the aircraft holding power for '20 seconds', was this perhaps in cool weather ? Certain types require a 'run-up ' at (e.g. 60% N1 - 1.2 epr) for 10 seconds if the aircraft has been deiced, or in severe icing.

Expedite climb.

The PIA t/o I'm referring to was in 30 degC conditions. I assumed it was because of the less dense air and minimal wind conditions that such a tactic was needed (albeit not by other 747's). As I stated earlier you don't have to visit an airport to witness their t/offs. You do occasionally see them some 20 miles from the airport still under full power and very low. It's clearly not right even for a non pilot. They must be overloaded or something. Surely ATC must have observed these t/offs and reported them to somebody?

Hey maybe these guys are burning off fuel. How many lbs/hr does 4- JT9Ds or 4- CF6s or 4-RB211s or for that matter 4 Russian equivalent Engines burn in 20 Seconds or 6 MINUTES AT MAX POWER AT SEA LEVEL?:O ;) :) :p

:uhoh: :{

ONLY TO FLY A jET:rolleyes:

SHEEP

And you are qualified to make such a statement???????


Mutt.

Hi,

The 747's will not be overloaded, but on most days they will be operating at full load.

BAe 146-100