Just After Takeoff
 

Hello

This is a question for all the pilots!!

After take-off the engines a reduced significantly, this is quite scary.

Do Pilots enjoy doing this, just to hear a few screams.

(Im no pilot or student, so please excuse my non-technical language)

Regards

Sam

"significantly"

Are you referring to the power reduction immediately after takeoff (30 seconds to a minute) or the reduction that occurs a bit later?

Go on take a guess....by what percentage do you think the total power is reduced by immediately after takeoff?

No, it isn't done to frighten the passengers, it's done for noise abatement reasons.

Taking a noise sensitive airport, such as Amsterdam Schiphol, the noise abatement procedure, which is typical of a lot of airports, is this:
Takeoff to 1500 ft above airfield: Takeoff power (which in most cases will not be full power).
1500 ft to 3000 ft above airfield: Climb power
At 3000 ft: Retract flaps and assume normal en-route climb.

The idea is to climb as steeply as possible close to the airport, commensurate with passenger comfort, and minimise disturbance to people on the ground.

There's a whole lot more to it than that, but does this sufficiently answer your question?

Thanx

Id say at least 50%, i probably wrong!!

What is it exactly?

Regards

Sam

this is from Avcom.co.za
 

A Novel Approach to Explaining How Aircraft Are Able to Fly
Most aeronautical engineers and the general public associate the lift generated by a wing with the differential pressure between the upper and lower surfaces of the wing. Nothing could be further from the truth. In reality, the lift required to make a commercial aircraft airborne is furnished by the passengers. Further, the lift is inversely proportional to both the wing size and the distance to be traveled. Farther further, the distance to be traveled has a non-linear relationship to lift, as will become clear in the following explanation.

1. How passengers provide lift for commercial aircraft: The lift required for an aircraft to take off is furnished by the passengers pulling up on their seat armrests.

2. How take-off lift is initiated by the pilot: After the aircraft reaches the end of the runway preparatory to take-off, the captain will advance the throttles on the engines. This action has two purposes: a) to provide horizontal thrust to propel the aircraft down the runway, and b) to increase the Passenger Aggregate Fear Level (PAFL) by raising the noise level in the cabin. The consequent rise in PAFL causes the passengers to strenuously lift up on their seat armrests, thus imparting lift to the aircraft. As we can readily see, the engines have two purposes, to move the aircraft horizontally and to scare the bejabbers out of the passengers.

3. How the duration and degree of lift is modulated by the pilot: Once cruising altitude is reached, the pilot will throttle the engines back to lower the noise level. The reduction in noise level results in a reduction in PAFL, with a consequent decrease in lift. It is necessary for the pilot to make only minor changes in noise level to maintain straight and level flight. In some instances where the PAFL does not decrease sufficiently to prevent further climbing, the captain may order that free drinks be passed around, thus further relaxing the passengers and lowering the PAFL.

One may observe that on most aircraft the first-class passengers are automatically anaesthetized by the use of free booze. Clearly first-class passengers are a source of surplus lift and must be dealt with accordingly.

While the airline industry will never admit it, passenger seating assignment is governed by national characteristics. For instance, Italian males are hardly ever upgraded to first class since they are easily excitable, respond very quickly to outside stimuli and provide almost immediate changes in lift. Clearly one would not want to get the Italians drunk. One difficulty associated with using Italians in this manner is their clannish nature; getting them evenly distributed (left and right, front and back) within the cabin can sometimes be difficult. Stewardesses will often resort to eyelash batting and hip wiggling to move the Italians about the aircraft.

While at first blush it may seem that the French would also be a good source of lift, their uncooperative nature makes lift modulation difficult. One should never fly on an aircraft containing more than 45 percent (by volume) Frenchmen.

The reader will note that Lufthansa, SAS and KLM fly only very large aircraft. Raising the PAFL for the stolid Germans, Swedes and Dutch is notoriously difficult, requiring as many people as possible in each aircraft. The British never fly.

The high takeoff-accident rate for Aeroflot can be attributed to the fact that Russians are generally drunk before they get on the aircraft and are not a reliable source of PAFL-induced lift.

Descent and landing are accomplished using a combination of fatigue and passenger discomfort. It is a happy coincidence that travel over greater distances takes a correspondingly longer time. Even the most casual observer will note that after the aircraft reaches cruising altitude the plane will begin a slow and gradual descent for the balance of the trip. This descent is due to passenger fatigue and discomfort. A detailed explanation of the fatigue factor is unnecessary; suffice to say that with time one's arms get tired and the upward pull on the armrests is reduced. By reducing leg and hip room, passenger discomfort is increased with time, and this distraction is also sufficient to reduce the Passenger Induced Lift, or PIL. The common airline practice of showing only the most boring of in-flight movies is also a lift-modulation technique.

Nota bene: The decrease in the amount and quality of airline food has not been found to be an effective method of PAFL modulation; biogas production offsets any decrease in lift. (See Hindenburg disaster, reference no. 75.)

Several recent instances of sudden aircraft descent have been attributed to air pockets. The air pocket explanation is clearly a feeble attempt on the part of the aircraft crews to avoid blame. In reality the crew neglected to closely monitor passenger fatigue, discomfort or degree of inebriation. Luckily sudden decreases in altitude are self-correcting due to the consequent rise in panic levels and increase in PAFL-induced lift.

Most passengers and the general public believe that the oft experienced practice of circling the airport many times prior to landing is caused by the weather. This is not wholly the case. During bad weather the PAFL increases as the aircraft reaches its destination. This undesirable increase in PAFL and consequent increase in lift must be dissipated by prolonging the flight and further tiring the passengers.

4. Historical basis for this theory and the role of PAFL in aircraft design: As your may recall from early aeronautical history, the Wright Brothers' aircraft had four wings with a very large surface area. The large surface area of the wings inspired great confidence in Wilbur and Orville, decreasing their PAFL and, as a consequence, decreasing the altitude and flight duration capabilities of the Wright Flyer. As aircraft design advanced, it was found that smaller wing surfaces inspired greater PAFL, with a resultant increase in aircraft performance. Indeed it was not until the advent of the multipassenger aircraft (with a higher PAFL factor) that increases in range and altitude were possible. The only reason wings (albeit very small ones) are still included on aircraft is that they look nice.

It is a little-known historical fact that the general unpopularity and eventual demise of the supersonic passenger aircraft were brought about by the fact that as soon as the aircraft reached supersonic speeds, the passengers could no longer hear the engines. No noise, no PAFL - and no PIL. The aircraft would drop like a rock, causing the PAFL to spike drastically, and the aircraft would then climb precipitously to a supersonic altitude, with a consequent loss of engine noise. The process would then repeat. The resultant sinusoidal altitude and speed changes have rendered supersonic travel impractical.

While further research by really annoying and pedantic people may bring my theory into disrepute, one must keep firmly in mind that even with all of the efforts to reduce personal space aboard commercial airliners, they have yet to remove the armrests. Think about it.

Here's a few reasons for thrust changes:

1: The aircraft may be in a lower class of airspace or traffic ahead and therefore may have a speed rectriction. This means that you do not need as much thrust to maintain a sensible rate of climb at the same speed. You would not enjoy sitting at the back looking up at a really high nose attitude and granny may s..t a brick. Think of your car going up a hill....you don't need to thrash the guts out to go 40mph.

2. As above in the last post, Noise is a major player in keeping granny in her garden happy and off that phone to ATC. Thrust is reduced to limit noise and air polution.

3: Engine management. With the cost of an engine these days, airlines cannot maintain poor economics. Would you thrash your car everywhere to the red line and trust it driving to France with the kids?
It would cost a fortune and probably go bang!

4: The thrust may be reduced when the aircraft has passed a strong inversion (turbulant or windshear layer due to a marked temp change) and a reduced setting can be used, again for engine life, noise and comfort.

5: If taking off with a tailwind (perfectly safe in the limits) then thrust may be reduced when at a safe altitude.

I think that this is the simplest that any of us can put it without any tech. input.

Re: Just After Takeoff
 

Nope, it's more like 10-15 % :ok:

Re: Just After Takeoff
 

Take off speed is not actually full engine power possible on the ground?

In other words when aircraft takes off are the engines at the highest possible power?

Re: Just After Takeoff
 

[QUOTE][In other words when aircraft takes off are the engines at the highest possible power?/QUOTE] Only if they have to be; say a shortish runway and a heavyweight aircraft. Otherwise we use any excess runway available to reduce the power (to save engine wear) and still reach the required speed to take off with the required safety margins.

PP

Re: Just After Takeoff
 

In fact, takeoff thrust may be reduced so much that as the plane is approaching cruise altitude, proportionately higher thrust (i.e. hotter turbine temperature) is used than at TO.

Re: Just After Takeoff
 

Little One - lovin it :) :)

Thanks for the laugh, all makes much more sense to me now!

cheers ;)
FF

Re: Just After Takeoff
 

The replies have missed the obvious. Take off thrust is applied at the end of the runway. There is a maximum time limit on the engines being run at this thrust level or they would break which would reduce the noise even more, although significantly increase the noise in the cabin! At between 800 feet and 1500 feet (dependent on company policy) the thrust is reduced to climb thrust. This level of thrust can be kept indefinitely without any risk to the engines.

Any further reduction inthrust due to noise abatement is another matter.

Re: Just After Takeoff
 

I am yet to work for a jet operator who reduces to/ beyond climb thrust for noise purposes?? All the noise sensitive airports require either a turn shortly after takeoff to avoid overflying the noise sensitve area, or else a climb to 1500' AGL before reducing to climb power and delaying flap retraction to around 3000' AGL. I am no expert, but I suspect maintaining as high a thrust setting as posible, and keeping the speed back decreases the noise footprint on the ground much quicker than reducing to a much lower thrust setting (much quieter onboard), whilst only just maintaining a rate of climb.

You are indeed correct as to why we reduce to climb thrust.

PP