Flaps a potential Achilles’ heel ?

[veldmuis]

Question from a non-pilot-dummy to the pilots out there on Luchtzak:
Are the tracks/racks of the flaps a plane’s potential Achilles’ heel?

During final approach the flaps are fully extended to maintain sufficient lift at low speed (and altitude). Then they are fully exposed to those strong airflow powers underneath, hence the ‘buffeting’ that passengers can see. The flaps are hanging from the wings on typically 2 tracks/racks each (aka “flap support structures”, such as produced by Belgian companies Asco, Watteeuw and Sonaca subcontracting for Airbus).
Imagine that right at such critical moment one of those flap support structures would be affected by a structural flaw (e.g. fatigue) so that the flap on one wing would come (half/entirely) loose. Or, less extreme, that the flaps on one wing would fully deploy, but on the other wing would remain stuck. The aerodynamic forces would immediately become non-symmetrical, causing one wing to drop.
Given the limited speed, height and thus reaction time available at that moment, would a pilot be able to recover the plane from such a dire worst-case situation? By very quickly retracting the other wing’s flaps – but without however getting into a stall at that low speed without any flaps deployed? By applying TOGO-power and increase speed – but without first worsening the non-symmetrical lift pressures even further? Or would simply (an extreme) aileron application be possible and actually have been designed to enable levelling the wings again?

Any pilots out there who have such a scenario in their training or manual for a certain type?
Thanks!

[Flanker2]

During the bijlmeer ramp, the flaps were lowered in preparation for approach. While the flaps on the left side operated fine, the ones on the right side were sheered off with the engines. This assymetric flap extension caused the B747 to get out of the controllable flight envelope and crash into the buildings.

Assymetric flap deployment is normally blocked by a mechanism that senses excessive torque on the torque tube which connects both flap mechanisms on the right and left side. But obviously, this requires the torque tube to be working and this is not always a certitude: https://www.youtube.com/watch?v=zMmM3sViycA

There may seem to be only few flap tracks on modern airliners as you count the flap track fairings but there is usually one track hidden at the wing root as well. The flap tracks are held in place by solid fasteners, so I don't see them getting loose without ripping off a piece from the wing. The tracks are very solid steel constructions. The flap carriage mechanism does seem to be the most vulnerable as it sits on the tracks thanks to rollers (a bit like a monorail) and together with the rotating spindle is prone to becoming stuck. I'm not fond of this kind of engineering, especially in today's world where we can work more precisely with bearings.

On the smaller Cessna's, the flaps look really vulnerable as they are moved by thin rods.

In my opinion, flaps were a tool intended to adapt the aircraft to the lack of adequate infrastructures, ie too short runways, back when they introduced the first commercial jets like the B707's. This feature could have been removed by adapting the runway infrastructures to the aircraft rather than the reverse. But they haven't.
The results is unnecessary weight, drag and aerodynamic inefficiencies, more complexity and maintenance costs. This could be much better compensated by lengthening and widening the runways, which is much cheaper.

[jan_olieslagers]

During the bijlmeer ramp

probably meaning "the Bijlmermeer disaster" - a ramp is not the same thing in English as it is in Dutch

were sheered off

probably meaning "were shorn off"

makes me feel like an Enigma machine. If the management of this forum insist on posting in English, why can't they require posting in proper English?

[jan_olieslagers]

In my opinion, flaps were a tool intended to adapt the aircraft to the lack of adequate infrastructures, ie too short runways, back when they introduced the first commercial jets like the B707's.

This is utter nonsense. Flaps were introduced much earlier - if Wikipedia is to be believed, one certain Mr. Handley-Page tried his hands on them in the 1920-1930s.

I am not wanting to disrespect your valued opinion - I must admit though to prefer factual information.

[jan_olieslagers]

Having done with the stupidities, let us consider the original question. An Achilles’ heel? Yes, surely. Aircraft have several. They become more numerous as the plane gets more complex. Only things that aren't there will never go wrong. Which is one reason for me to prefer flying an extremely simple craft, some pilots might well name it spartan.

For one example, my fuel is fed to the engine by sheer gravity, there is no electrical fuel pump. Gravity has been shown statistically to be a zillion times more reliable than anything electric, so I am on the safe side there. Which still leaves many many other things left to break and spoil the fun of flying, and likely much more.

But the most dangerous moments of any flight are "just after take-off" and "just before landing". Speed is minimal, then, so the slightest imbalance will have maximal repercussion. Forces related to airspeed are generally relative to the square of said airspeed, so that at half the airspeed the ailerons have only a quarter of their maximal effect. Anything breaking in the low airspeed phase will demand maximum pilot performance, and there are situations where even that will not save the day. As in the sad case you brought up.

[teddybAIR]

The Achiles' heel of any aircraft is the pilot/crew in front. Hence the emphasis on CRM and increased interest human performance & limitations the last few decades.
Flap tracks or racks don't simply fail for no apparent reason. You would have to fly the aircraft significantly above the flap extention speed for a specific flap setting and it would require a number of redundant systems to fail as well. You mention the case of fatigue...not a single flap will separate if people (we) perform our maintenance procedures as prescribed. That brings us back to human performance.

So to answer your question: no, flaps are not the achiles' heel. The inconvenient truth is that we are

[Stij]

In my opinion, flaps were a tool intended to adapt the aircraft to the lack of adequate infrastructures, ie too short runways, back when they introduced the first commercial jets like the B707's. This feature could have been removed by adapting the runway infrastructures to the aircraft rather than the reverse. But they haven't.

Flanker2,

You made my day! Let's extend 25R up to Kampenhout Sas and we're all fine!

Seriously, I don't know if your theory makes sense from a scientific point of view but I do know that:

If every runway should have been lengthened for every bigger plane that came on the market, nobody would have bought the plane, because there wouldn't be any airport to fly it to.

Example? This is the very reason Siemens can't sell it Maglev train: it isn't compatible with current infrastructure...

Cheers,

Stij

[teddybAIR]

The fact of the matter is that flaps are designed to:
> Adapt high speed aerofoils for low speed flight and delay boundary layer separation (read: decrease stall speed)
> reduce final approach speed and thereby the kinetic energy that needs to be disipated by arresting gear such as brakes & spoilers
> Reduce Landing Distance Required to an operationally sensible distance (cfr. Flanker2)
> increasse drag on final approach to be able to spool up the engines to approx 50% N1 and thus avoid long spool up times in case of missed approach/go around

So what Flanker2 mentions is one of the reasons why we fit aircraft with flaps.

KR,
bAIR

[Flanker2]

The best example is the NASA Space Shuttle. It doesn't have flaps, yet it lands fine on runways which have been designated as able to receive it.

Flanker2,

You made my day! Let's extend 25R up to Kampenhout Sas and we're all fine!

Seriously, I don't know if your theory makes sense from a scientific point of view but I do know that:

If every runway should have been lengthened for every bigger plane that came on the market, nobody would have bought the plane, because there wouldn't be any airport to fly it to.

Let's take BRU as example and try to quantify the inefficiency caused by flaps. Let's say that 200 narrowbody flights and 20 widebody flights land at BRU every day.
How much is the cost of carrying flaps for all those aircraft? Between flap tracks, flap assemblies, fairings, motors/actuators, hydraulic lines, control computers, spindles, etc...you can count over a ton of weight for narrowbodies and a couple of tons for widebodies.
Add aerodynamic inefficiencies during the entire flight including the phase where the flaps are deployed as the flap's drag must be countered by more thrust and hence more fuel flow, but also in cruise where flaps cause drag in many different ways (just look at those huge fairings sticking below the A330's), and we're talking about a fuel penalty of XX thousand of Euro's for BRU alone every single day.

On a yearly basis, this penalty becomes X or XX millions of euro's.

This is only fuel burn, we are not counting the maintenance cost, design and manufacturing cost, training cost, etc...

That's plenty of money to cover for a runway extension, don't you think?
In BRU for instance, if it was necessary, they could easily extend and widen 25L for landing aircraft.
25R could easily be used as is for smaller landing aircraft and take-off where margins permit.

But why should airports bother investing in more runway infrastructure if airlines are happy to buy aircraft equipped with flaps?


It's no longer possible to remove the flaps from a design point of view because too many airports would need to be adjusted, and it would be impossible to manage traffic efficiently in a combination of flaps equipped aircraft with non-equipped aircraft. This is why aircraft manufacturers are considering alternatives such as active wing technology.

I think that many people take flaps for granted but the truth is that flaps which were intended as a temporary solution for too short runways ended up creating a permanent problem.

It's no stupidity, flaps are one of the major inefficiencies of our modern commercial aviation. In fact, if you look at the B787 and A350XWB, you can see how much effort Airbus and Boeing have made to reduce the size of the flap track fairings.

This is no theory, it's fact.

Flaps cause a lot of maintenance costs. They need regular greasing at the carriage assemblies, at the spindle.
They sometimes need adjusting when they become unstable.
They can loose flap track fairings.
They can and do get stuck, causing emergency landings and lost revenue.

Flaps have caused accidents and aircraft losses such as China Airlines 120. http://en.wikipedia.org/wiki/China_Airlines_Flight_120

[teddybAIR]

How much is the cost of carrying flaps for all those aircraft? Between flap tracks, flap assemblies, fairings, motors/actuators, hydraulic lines, control computers, spindles, etc...you can count over a ton of weight for narrowbodies and a couple of tons for widebodies.Add aerodynamic inefficiencies during the entire flight including the phase where the flaps are deployed as the flap's drag must be countered by more thrust and hence more fuel flow, but also in cruise where flaps cause drag in many different ways (just look at those huge fairings sticking below the A330's), and we're talking about a fuel penalty of XX thousand of Euro's for BRU alone every single day.

Hi Flanker,
Valid argument on the weith penalty of flaps, but unfortunately, you 'forget' to take into account the benefit that thanks to the fact that an aircraft carries flaps, the wing can be designed for higher cruise speeds compared to a wing without flaps. Do you really think that aerospace engineers entrusted with aircraft design do not optimize the design of their aircraft taking into account factors you couldn't even imagine in your wildest dreams.

In your logic, it wouldn't be beneficial to carry winglets either as they represent a weight penalty! So do not forget to make the entire equation. Is it that hard to simply trust some of the most advanced aeronautical specialists whose million dollar bonusses depend on fractions of percentages of savings in terms of opperating costs?

The danger of forums such as these is that some members like to position themselves as aeronautical experts while they probably didn't even follow a PPL-level aerodynamics course. Consequently they focus on secundary effects such as the parasite drag of fairings around flap tracks while forgetting design benefits from designing an optimum wing for design speeds at cruising altitude.

But rest assured: the person on this forum that would be able to improve an aircraft design in such a manner that it would shave even 0,1% of the opperating cost will be a very wealthy man

[Flanker2]

Valid argument on the weith penalty of flaps, but unfortunately, you 'forget' to take into account the benefit that thanks to the fact that an aircraft carries flaps, the wing can be designed for higher cruise speeds compared to a wing without flaps. Do you really think that aerospace engineers entrusted with aircraft design do not optimize the design of their aircraft taking into account factors you couldn't even imagine in your wildest dreams.

I think that I must tell that I have studied aircraft design to some degree.

A wing made of a single piece optimised for high speed subsonic flight is much better than a wing with flaps optimised for the same speed. Design engineers would very much prefer to be able to design wings without flaps, but they can't because the industry is not prepared for it.

The requirement to install flaps forces engineers to adapt their wings from optimum for the flaps.
Flaps result in small slots between the main wing and the flaps during cruise, resulting in drag and loss of lift, in addition to the fairings' huge drag.

So unfortunately, it's not up to the engineers to decide whether they should integrate flaps into a new design, it's an industry effort. Engineers are now trying to take the best of both worlds by integrating active wing technology, which would make the main wings take over the functions of the flaps.

[CaravanDriver]

Without flaps and slats the take off and landing speeds will be to high to be safe. On the aircraft I fly a landing at max landing weight , 140T, it results in an approach speed of nearly 190Kts. The max tire speed is 195Kts. You never can design a wing that can cope with high and low speeds together.
In the take off case it is known that the higher the speeds the more dangerous it gets. Imagine the abort after engine failure just before V1. In our case at high weights V1 is around 165Kts. You really think you. An design brakes that can cope with a emergency stop of an 186T aircraft at speeds well above this 165Kts? It is presently already very criticale to stop.

[jan_olieslagers]

Also: if flaps are only there to reduce the required runway length, why were they also installed on the Short Sunderland? Being a seaplane, it had virtually unlimited runways available. Neither did it have tire speeds to respect.

[Flanker2]

Without flaps and slats the take off and landing speeds will be to high to be safe. On the aircraft I fly a landing at max landing weight , 140T, it results in an approach speed of nearly 190Kts. The max tire speed is 195Kts. You never can design a wing that can cope with high and low speeds together.
In the take off case it is known that the higher the speeds the more dangerous it gets. Imagine the abort after engine failure just before V1. In our case at high weights V1 is around 165Kts. You really think you. An design brakes that can cope with a emergency stop of an 186T aircraft at speeds well above this 165Kts? It is presently already very criticale to stop.

Yes you can. Max tire speeds are irrelevant as tires can be designed for higher max speeds.
They are not on commercial aircraft, in the interest of saving weight. Something has to make up for the flap's weight, you know?
The Space Shuttle landed at 200kts, yet weight is very critical and expensive for any spacecraft. Every additional excess weight of 100 kg added 2 millions dollars to the cost of a mission.
Many supersonic-capable aircraft also had high approach and landing speeds, because well, they are optimised for light weight and high speed flight, while infrastructure was tailored to them rather than the aircraft tailored to the infrastructure. You don't have double-slotted Fowler flaps on the F16, do you?

The situation is simple to explain using the British naval carriers that aren't equipped with arrestor cables and catapults. The British decided that their naval fighter of choice would be the Harrier, and hence designed the naval carriers at lower cost, without arrestor system and launch system. The result is that they are stuck and will remain stuck operating VTOL fighters with less fighting capability, the very purpose they are designed for, against the merits of designing a more expensive but capable aircraft carrier infrastructure.
They will hence get stuck operating less capable (in combat) F-35B's where the U.S. will be able to use F-35C's.


Your abort after V1 case is irrelevant if the infrastructure is in place. If you have sufficient runway length, you can bring it to a stop without having to design heavier brakes. In fact, if you have sufficient runway, V1 speed is no longer a factor at all, as it will coincide with VR.
How many tons is irrelevant really, it's not like an A380 is fitted with the same brake system as the A320.

Jan, seaplanes have different landing requirements.
Despite what you think, STOL are often part of the requirements of seaplanes.
The Shorts Sunderland was not a modern jet commercial aircraft where efficiency was a main priority.
Designing tires and wheels to withstand higher landing speeds is one thing, designing efficient flying boats with hulls that can withstand 150kts landings on 5 foot waves is something else.
Leaving aside water landing and tkof characterisitics, seaplanes are also very often designed with STOL characteristics, and not under the assumption of limitless infrastructure, like you seem to assume. Simply because the "limitless infrastructure" is very difficult to achieve, often much more difficult than lengthening runways. Just think about ship traffic in and around ports, obstacle clearance around lakes, high waves in open waters, etc...

[teddybAIR]

I thought the discussion was about whether flaps are a potential achilles' heel. Can we agree they are not? Or should I open another thread with the question weather retractable gears are a potential achilles' heel? Let's do the same for spoilers & let's not forget the wings either as they cause great concern for obstacle clearance...

[CaravanDriver]

Keep on dreaming of your airliners without flaps. Operational it is not possible. The list of reasons is even to big to mention.

[Flanker2]

I think that future desing engineers will realise that the drag, weight, loss in fuel tank capacity, mechanical and engineering complexity and resulting costs of carrying flaps opens the door to a major rethink of the concept.

Call me crazy, but a change in future propulsion fuel and their (lower) energy density will probably also bring a drastic change in wing design considerations.

[Stij]

Could be, but the runways will stay the +- same!

Cheers,

Stij

[teddybAIR]

I'm not sure about the weight penalty. The higher take-off and landing speeds would require increased strength of the landing gear assembly + higher rated speeds of the tires. Increased strength comes at a weight penalty as well, so although you would save weight of the flap assembly, you would increase the weight of other components. Believe me: in the past decades, some really specialised people who don't even have the time to waste in this type of threads have made these trade-offs countless times!