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Flaps Extension During Cruising  
User currently offlineCricri From France, joined Oct 1999, 581 posts, RR: 7
Posted (15 years 1 day 5 hours ago) and read 3821 times:

What could be the possible reason for a flap extension during the cruisin of a commercial jet?
It happened to a friend of mine two days ago on board an A340-200. Had nothing to do with taking-off, approach or landing phase.
I know one reason but it is unofficial because it is forbidden : when a plane is at its max. cruisin altitude and it has turbulences on his flight level, some crew during the past, put just a few degrees flaps by starting to extend them and immediately after put the breaker out. This allows the plane to climb a few feet higher with only 2 or 3° flaps. Can a pilot confirm it to me please?
So to my first question, is there another explanation for this unusual extension?
Many thanks in advance and best regards.

16 replies: All unread, jump to last
User currently offlineIainhol From , joined Dec 1969, posts, RR:
Reply 1, posted (15 years 1 day 4 hours ago) and read 3675 times:

They would not do it to climb. They can go full power and climb. They probably did it to slow down and stablize. We never put flaps down to climb!

User currently offlineOPNLguy From , joined Dec 1969, posts, RR:
Reply 2, posted (15 years 1 day 4 hours ago) and read 3669 times:

Just my opionion, of course, but I think that's an industry myth dating from Hoot Gibson's TWA 727-100 "death dive" from FL390 years ago. It was postulated that they pulled the C/B for the trailing edge flaps so they could get 1 or 2 degrees of L/E flaps/slats and additional lft, but darn if the incident aircraft didn't have an uncommanded slat deployment within a year or two prior to Gibson's flight.

Aircraft flaps have speed limits for extension and retraction, at at normal cruising speeds (at altitude) you're exceeding their maxium extension speed.

User currently offlinePanman From Trinidad and Tobago, joined Aug 1999, 790 posts, RR: 0
Reply 3, posted (15 years 1 day 3 hours ago) and read 3665 times:

The only reason I can think of for the flap extension would be to slow the aircraft down.

Flaps work by increasing the area of a wing (and to some extent it's camber) which gives an overall increase in lift. It also gives an increase in drag (you see this by comparing the formulae for co-efficient of lift and co-efficient of drag).

The drag in this case would have been enough to slow the aircraft down quicker than just by reducing power and lifting the nose to compensate.


User currently offlineCricri From France, joined Oct 1999, 581 posts, RR: 7
Reply 4, posted (15 years 1 day 3 hours ago) and read 3658 times:

Cool, I didn't know about this method to slow down the aircraft ; is this common? Thanks very much for your answers.

User currently offlineJETPILOT From United States of America, joined May 1999, 3130 posts, RR: 29
Reply 5, posted (15 years 1 day 3 hours ago) and read 3668 times:

Extending the flaps in turbulence would be suicide. G load limits are reduced when the flaps are extended.

Aircraft also have a max flap extension altitude. The 727 is FL200. Above that altitude the flaps would react differently in the thinner air.

Lift does not make an aircraft climb. Excess thrust is responsible. During a climb the vertical component of lift opposes the directrion of flight working to slow the aitcraft. The thrust vector is pointed downward causing the aircraft to climb.

Every transport category aircraft uses flaps to climb. Usually 10 -15 degrees on TO.

You would find no such procedure for flap extrension at cruiuse in an aircraft operating manual. Therefore you are essentially a test pilot when doing so endangering the lives of the passengers and crew.

Not to mention that the aircraft would have to be slowed below max flap extension speed before being deployed. Controllers woud never allow an AC to be flying so slowly in the J routes.

Sounds like a line of BS to me.

Flaps don't slow aircraft down either. They permit a steepere angle of descent for a given airspeed. Most importantly they increse chord line. Secondly they increase area if they are fowler flaps. Not al AC have a fowler flap. The DC8 is one that comes to mind.

User currently offlinePhil330 From Australia, joined May 2011, 0 posts, RR: 0
Reply 6, posted (15 years 1 day 3 hours ago) and read 3658 times:

Flaps are never used to slow the aircraft down, our manuals all state that "Flaps should never be used as speedbrakes". The stress put on them is unacceptable under these circumstances. We never extend flaps during cruise, this can have fatal consequences.

A320/330 pilot.

User currently offlinePanman From Trinidad and Tobago, joined Aug 1999, 790 posts, RR: 0
Reply 7, posted (15 years 1 day 2 hours ago) and read 3642 times:

I agree in that flaps are not used to slow aircraft down. I was suggesting a possible reason for the usage as it was mentioned by the author of the original post.

An extension of flaps will slow an aircraft down as it will increase drag (see later on).

You will also note that I said they increase drag and lift which they do. They allow you to fly slower without stalling due to the increased lift.

I never said lift will make you climb. And I disagree with you JETPILOT to some extent on your statement that thrust makes you climb. It is ovbvious you are aware of your four forces in flight (lift weight thrust drag). In straight and level flight all four forces are in equilibrium. For an aircract to climb lift must overcome weight, for an aircraft to move forward thrust must overcome drag. Lift is not produced unless an aircraft is moving forward (we are not talking helicopters here). Lift does not 'oppose the direction of flight' as the direction of flight is not directly upwards or downward but forwards. Lift opposes weight. It is basically the effect of lift/thrust overcoming weight/drag that makes you climb.

You will note I also said they increase camber to some extent, this causes a greater pressure differential (venturi effect) as the air is speeded up more than normal, this gives greater induced drag and will slow an airplane down.


(PS I think this is coming out disjointed, I know what I am trying to say, it's trying to get my thoughts to come out in a logical order)

User currently offlineBruce From United States of America, joined May 1999, 5067 posts, RR: 15
Reply 8, posted (15 years 1 day 2 hours ago) and read 3636 times:

Are you sure you meant the FLAPS were extended on your friend's flight? Maybe you meant the SPOILERS? Were they the panels on top of the wing? A pilot could deploy the SPOILERS slightly at altitude as a means of slowing the plane without reducing engine speed. Especially if he's preparing to descend.

that happened to me on a 757 flight at FL260.....spoilers came up part way.

Bruce Leibowitz - Jackson, MS (KJAN) - Canon 50D/100-400L IS lens
User currently offlinePhil330 From Australia, joined May 2011, 0 posts, RR: 0
Reply 9, posted (15 years 1 day 1 hour ago) and read 3638 times:

In fact inceased thrust does make an aircraft climb, in the same way that thrust reduction makes it descend, pulling up decreases speed and pushing forward increases it.

On final approach to land on any aircraft excessive speed is corrected by pulling up, a low speed is corrected by lowering the nose. In the same way being too low on the glideslope is corrected by increasing thrust and being too high is corrected by reducing thrust.

The 'two sets of two' forces argument is not true at all, each complements each other. You're statement implies that the only way lift is increased is direct (i.e. wing angle of attack increased or weight decreased) although thrust has precisely the same effect. This is caused by an increased speed (increasing lift) to some extent and on some aircraft (like the 737) the powerplants are so much lower than the C of G that increases in thrust provide high pitch changes.

I do not disagree with your comments directly, and am aware that drag caused by flaps slows the aircraft down, but my point was that they are never used for this purpose.

A320/330 pilot.

User currently offlineJETPILOT From United States of America, joined May 1999, 3130 posts, RR: 29
Reply 10, posted (15 years 1 day 1 hour ago) and read 3635 times:

If you tilt the Aircraft up into a climbing attitude you will notice the lift vector which always acts perpendicular to the surface of the wing is now working in a rearward vector. So lift does oppose the direction of flight in a climb. Lift only opposes weight when the aircraft is flying stright and level. The four fources of flight are a general rule but are bent when the aircraft is moved out of a straight and level attitude.

A perfect example of a excess thrust causing a plane to climb is illustrated by military fighter jet and thier ability to climb straight up. No lift needed there. The same is true for airliners and all other AC but to a much lesser extent.

User currently offlinePanman From Trinidad and Tobago, joined Aug 1999, 790 posts, RR: 0
Reply 11, posted (15 years 1 day ago) and read 3624 times:

But you guys are still wrong in a sense. All the thrust component does is propel the wings through the air which causes lift. The faster you go the more lift is produced, the slower you go the less lift is produced. If you lift is reduced then it's obvious that weight will be the greater of the lift/weight components and the aircraft will drop. Conversely if thrust is increased, lift is increased - lift then overcomes weight and the aircraft climbs. That gives you the reason why the aircraft will drop/rise when thust is increased/decreased. The thrust is having a direct effect on the forward speed of the aircraft and thus the amount of lift that the aerofoils (wing, horizontal stabilizer and to a small extent the fuselage - look at it, it is shaped like an aerofoil) will produce.

Also JETPILOT when you tilt the aircraft lift does not 'move' rearward. Lift is still acting upwards and weight downwards, it's the aircrafts position relative to these forces that changes.

Having said all of that though all of us have been arguing a moot point as lift or any of the other four components do not in themselves govern the rate of change of altitude but it is a combination of ALL four. You remove thrust then there is nothing resisting the drag component (the reciprocal of thrust). The fact that drag is the greater of the T/D couple means that the forward speed will decrease. This decrease in forward speed means that lift now decreases as lift is a direct result of forward speed. As lift now lessens this means that weight is the greater of the L/W couple. The natural reaction therefore with the decrease in thrust and lift is for the aircraft to obviously begin to descend. (I think my Theory of Flight lecturer would be glad to see me arguing this point even if it gets proven that some of my points are wrong).

Hope this isn't getting too technical now.


User currently offlinePhil330 From Australia, joined May 2011, 0 posts, RR: 0
Reply 12, posted (15 years 1 day ago) and read 3627 times:


I did not state that any of your post was incorrect, nor did I prove any of it incorrect. All of what you say is correct, a very good understanding of the principles (whih I learned too many years ago). The basis of my argument was that you implied that changing thrust does not result in changing lift at all, and that you were not taking the changes in airspeed. I emphasised my point by stating that the way we alter lift on approach (vertical speed) is not using pitch commands but thrust commands.

I belive JETPILOT was stating that the perpendicular lift force meant that during turns and pitch alterations the lift acts in a different direction to the normal (perpendicular to the wing, therefore no longer directly up). During a pitch increase some of the lift force is made up of a rearward force component as well as an upward one.

A320/330 pilot.

User currently offlinePanman From Trinidad and Tobago, joined Aug 1999, 790 posts, RR: 0
Reply 13, posted (15 years 1 day ago) and read 3620 times:

I agree I re-read my earlier post and it did seem as if I was not taking into account the thrust component. Like I said in one of them, I know what I mean to say, it's just how I put it down on paper.

User currently offlineJETPILOT From United States of America, joined May 1999, 3130 posts, RR: 29
Reply 14, posted (15 years 22 hours ago) and read 3617 times:

If lift makes an airplane climb then can you explain to me why a Cessna 172S (160 HP) and a 172R (180 HP) have different climb rates. Both are based on the same airframe. The 172R climbs about 200-300 ft per minute better than the 172S.

Weight always acts perpendicular to the gorund. Lift acts perpendicular to the wings surface. The yoppose each other at varying percentages depending on the angle between the surface of the wing and the ground which is variable with aircraft attitude.

Here's a simple example. Take a piece of paper and draw the 4 forces of flight. Now rotate that aircraft to a nose high attitude and notice lift does act in a rearward axis. And in return thrust in a climb opposes weight. There is really nothing to argue here.

Phil330 is on the same page as I am.

A brich will fly through the air if it is given enough thrust.

Every action has an equal and opposite reaction. If the thrust vector is pointed down then the reaction is for the mass to react in the opposite direction.

User currently offlinePanman From Trinidad and Tobago, joined Aug 1999, 790 posts, RR: 0
Reply 15, posted (15 years 12 hours ago) and read 3596 times:

Am I right in saying that the 172S has fixed landing gear and the 172R has retractable landing gear? If so therein lies the answer to your question. You have extra drag acting on the 172S and of course you are going to get different climb rates.

Lift does not act perpendicular to the wing surface at all times. What acts perpendicular to the wing is the Total Reaction force. If you plot the lift force and the drag force on a graph and then plot the resultant vector you get the Total Reaction force and this is always perpendicular to the chord of the aerofoil.

If I remember correctly from Theory of Flight (it's been over a year now), Lift is the component of the total reaction force that acts at right angles or perpendicular to the RELATIVE AIRFLOW. In this respect you can get lift that is not acting straight up. The only time therefore that lift is perpendicular to the wing is when the relative airflow is is parallel to the chord of the wing.

Drag is the component of the total reaction force parallel to the relative airflow and opposing motion.

Also if as you claim lift acts perpendicular to the wing then when a aircraft is in cruise it should be constantly wanting to tilt nose up as most aircraft have their wings set at an angle of incidence of 3-4 degrees so that when in cruise (straight and level flight) they have a positive angle of attack and generate lift and keep the aircraft from falling out of the sky.


User currently offlineCricri From France, joined Oct 1999, 581 posts, RR: 7
Reply 16, posted (15 years 11 hours ago) and read 3595 times:

OK folks, I learned a lot by reading your posts regarding to what happened on my friends flight and I thank you for this even if it was a little bit hard to understand everything. I phoned him back yesterday evening (I'm on GMT+1) to know if he was really meaning the flaps and it is so. He told me that it was so unusual that the f/as get very surprised (and so the trolley that made a trip trough the cabin  ). He noticed them extended on the wings (a little less than half way and the plane slowed down so the engines sound) and saw them retracted after a couple of minutes. Immediately after retraction, the plane recovered it's normal cruising and engine sound that it is used to hear. That's all I have for you.

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