I don't agree with this FAA ATPL Question
 

Hello from the states. I'm doing some review for an upcoming interview and I was reviewing my FAA ATP material.

There's one question and answer that I don't agree with. I wrote the FAA two weeks ago about this question (because I have no life) and never got a response.

FAA Airman's Information Manual (AIM) 7-3-3 and FAA ATPL question 9119:

"Which flight conditions of a large jet airplane create the most severe flight hazard by generating wingtip vorticies of the greatest strength?

A. Heavy, slow, gear and flaps up.
B. Heavy, slow, gear and flaps down.
C. Heavy, fast, gear and flaps down."

The FAA says the correct answer is A.

However, I disagree. I think the correct answer is C because when a plane has it's flaps down, the plane is generating more lift and drag, thus creating a greater disturbance in the air. Also, with the gear down, there is more turbulence from the disturbed flow off of the gear. Finally, when a plane is traveling fast, the wing is generating much more lift than when the same plane would be flying slower.

Does anyone agree with me? What are your opinions?

Thanks.

It really doesn't matter, if the FAA say A Just put A then let experienced guys teach you how it should be done,

This is a function of the wing. Disregard the gear position.
The strength of wingtip vortices is a function of angle of attack - large AoA leads to stronger vortices. Which conditions give greatest AoA?

Heavy and slow or heavy and fast? Flaps up or flaps down?

Answer A is correct.

Once you get that job this will be backed up by real world experience when you're asked to maintain 230 knots and you're following a 757 that's clean.

The strength of the vortex is lift of which AOA is just one component.

Factors which effect lift are air density, circulation, wing area and velocity of the relative wind.

Therefore, assuming density plays no factor, the circulation will increase with flaps down as will the wing area, thus producing greater lift and stronger a stronger vortex. Also, a higher relative wind means the lift on the wing will be stronger, thus a stronger vortex.

Correct?

When a wing generates aerodynamic lift the air on the top surface has lower pressure relative to the bottom surface. Air flows from below the wing and out around the tip to the top of the wing in a circular fashion. An emergent circulatory flow, vortex is induced. Slow and flaps up = larger AoA, then larger pressure difference between lower and upper surfaces.

Not correct.
The lift is constant = weight, larger wing(flaps down) = lower delta pressure required, then less "leakage" from lower to upper.

I think you should figure out why this statement is incorrect first. Understanding lift vs weight vs air speed, and the mechanism through which the lift is generated is more important that a specific issue such as vortices and configuration.

I agree with the OP, that's what my study books said.

FD.

answer

D. There is no such thing as wingtip vorticies. :}

The gear down affects the rollup of the vorticies....so less vortex creation...while the Lazy B doesnt agree, there is lift from the center wing...

Gear down mitigates votexes but to a negligible degree. And yes being clobbered behind a clean 757 teaches one about wake turb very quickly! :uhoh:

It's 'A'. Read what Phoenix has written.

unless it is an A388 or B748...

Much better chance of getting clobbered if you are answer A for certain!

I was at the wakenet US, the FAA presented the new guidance for wake encounter mitigation for drivers...
The guidance made the mistake of saying watch the leader depart, and rotate before that point....the rep for A went ballistic...yelling, "you rotate when the ac tells you to rotate"...that guidance is bull****.... oh well.

on the short final issue, if you are that close, technically, wouldnt your FAF move to the beginning of the turn, so that you still have decision and the stabilized approach? FROP is 500 feet min, but you also must have 30 seconds of stabilized approach....FAP would be where you intercept the glideslope, and FAF is not coincident?

The gear position is to be disregarded, it is testing your understanding of flaps up AoA (and therefore additional lift, stronger vortices) vs flaps down.

Wrong!
 

You are alarmingly incorrect. Going faster or putting out flaps does not generate more lift - not if the yoke monkey on board is maintaining level flight..... Think about it....

More flaps or higher speed will indeed create more drag, and I suppose 'disturbance through the air'. However this question is limited to one specific aerodynamic phenomenon.

This is all about angle of attack. With flaps out you have larger wing area, therefore you reduce angle of attack to maintain level flight .This creates less pressure under the wing and hence a weaker vortex. Going faster also requires a lesser angle of attack to achieve the same lift, with the same result .

The strength of Wingtip vortex is proportional to the value of Induced drag. Ask your self what is Induced drag and when is it the highest and you 'll understand the question.

They use this as ref:


2.2 Historical Examination of the Wake- Turbulence Hazard
Wake turbulence is a natural by-product of powered flight, but was not generally regarded as a serious flight hazard until the late 1960s. Upsets or turbulence encounters associated with other aircraft were usually accredited to “propwash” and later on, with “jet wash.” Interest in this phenomenon greatly increased with the introduction of large, wide-body tur- bojet aircraft during the late 1960s and a con- cern about the impact of greater wake turbulence. This was the impetus to conduct research to gain additional information and determine what safety considerations were necessary as more and more large aircraft entered the industry fleets.
An investigation of the wake-turbulence phe- nomenon, conducted by Boeing in mid 1969 as part of the FAA test program, included both analysis and limited flight test and pro- duced more detailed information on wake vortices. The flight tests provided a direct comparison between the B-747 and a repre- sentative from the then current jet fleet, a B- 707-320C. The smallest Boeing jet transport, the B-737-100, was used as the primary wake- turbulence probing aircraft along with an F- 86 and the NASA CV-990. Smoke generating
towers were also used to observe the wake turbulence generated by aircraft as they flew by. Several observations were made.

• The strength of the wake turbulence is governed by the weight, speed and wing- span of the generating aircraft.

• The greatest strength occurs when the generating aircraft is heavy, at slow speed with a clean wing configuration.

Initial flight tests produced sufficient infor- mation about the strength, duration and move- ment of wake turbulence to come to conclusions and recommendations on how to avoid it. The wake was observed to move down initially and then level off. It was never encountered at the same flight level as the generating aircraft or more than 900 feet be- low the generating aircraft. Therefore, a fol- lowing aircraft could avoid the wake turbulence by flying above the flightpath of the leading aircraft. While this can be accom- plished in visual conditions, an alternative was developed for instrument meteorologi- cal conditions. Aircraft were placed into cat- egories determined by their gross weight. It was noted that a division based on the wing- span of the following aircraft was a more technically correct way to establish catego- ries; however, it did not appear to be an easily workable method. Since there is a correlation between aircraft gross weight and wingspan, gross weight was selected as a means of cat- egorizing aircraft and wake-turbulence strength. Minimum radar-controlled wake- turbulence separation distances were estab- lished for following aircraft. The separation distances depend on the weight of both the leading and following aircraft. Adjustments in separation distances were made as more information on the wake-turbulence phenom- enon was gained during the 1960s, 1980s and 1990s, but the basic concept of using aircraft weights remained constant.

Flaps and gear
 

Assuming the same aircraft weight and speed, A and B will generate similar initial vortices but B's will dissipate more quickly. To quote the FAA ,

Heavy, slow and clean is when an aircraft produce more wake turbulence.

.............

Interested pax.

At slower speeds the volume and mass of air swept by the wing in unit time is reduced. To maintain 1g you need to generate a fixed rate of change of momentum and with less mass to deflect you must induce a higher downward speed change to the air.

With flaps deployed speed needed to maintain 1g will be reduced so at first sight would expect higher circulation again but the wing has changed and isn't uniform along the span. The outboard section is unchanged but without flaps you couldn't maintain 1g. So the lift generated by the outboard section is reduced compared to the clean configuration.

Hope this didn't annoy......

flap setting are everything, even on the same variant,
it also depends on the crosswind component,
or if the flap edge discontinuity is ingested into the vortex or not...

ingested..adds to the rollup velocity from the small core outward...strong initial vortex, may dissipate faster....
http://i68.tinypic.com/28whyle.jpg

not ingested...large core vortex is not accelerated by discontinuity vortex...not as strong as above, but may be longer lasting
http://i67.tinypic.com/vpx5yo.jpg

then there is crosswind, just like the FAA says, the vortex is generated normally, then drifts away from the flightpath.. really?
reality...one really strong vortex, not coupled, on its own..VERY strong vortex...long lived and indeterminate behaviour

http://i67.tinypic.com/2qlaook.jpg