Differential friction

[runway2]

Hi, does anyone know how the aircraft would react to the presence of differential friction on the runway, where one side is subjected to lower friction due to ponding etc.. Is there any permissible yaw angle (for a given speed) that the aircraft could turn during the landing and regain control? thanks

[Intruder]

For large airliners, the antiskid braking system takes care of most of the problem.

For the rest, and for airplanes without antiskid systems, yaw would be the result, and corrections are made with rudder and differential braking as necessary.

There is no defined yaw angle for control. Yaw angle, yaw rate, braking effectiveness, and runway surface all will affect controlability.


Also, there is no need to send PMs on initial inquiries. Not only will you get broader responses in the public forum, but you will irritate those to whom you send the extra PMs, and likely cause them to ignore you completely in the future.

[runway2]

thanks,

but tentatively for a pilot what would be the yaw angle at which there will be concern on whether the aircraft could veer off the runway?

[Intruder]

The answer is the same: There is NO DEFINED YAW ANGLE FOR CONTROL. Yaw angle, yaw rate, braking effectiveness, and runway surface all will affect controlability.

[777AV8R]

Inexperience....

If you have to ask this question, this shows your total lack of understanding about the handling of the aircraft. As in the previous post, there is no 'defined' angle. If you have to start asking that question then obviously you've already exceeded the limits for it or have gotten into something way beyond your capabilities.
ANY angle, combination of wind, drift, mis-alignment of the aircraft is cause for concern. Best you fly on CAVOK days until you get some understanding.

[Bullethead]

Even if there was a defined yaw 'limit' who is gunna be looking at it when you're sliding along a saturated runway trying to keep things under control.

I can just imagine,

"Oh look we're one degree less than the limit it's gunna be alright!"

Zero would be my limit, if it starts to yaw fix it and pronto.

Regards,
BH.

[runway2]

thanks for the comments, true i have zero experience
just wanted to find out under what scenario an aircraft would veer off a runway due to differential friction/ i guess u guys do not know that

[Rainboe]

We do know that, thankyou. It is not from an instrument giving you a compass heading from which you would have to subtract the runway heading to work it out at a time like that! There is no limit. It depends on the circumstances of the day and the pilot flying. But before it gets to that stage, it is extremely likely that ATC would have closed the runway- they do monitor runway friction and take action! Runway friction measurements are made and passed to tyhe tower- if lower than a certain limit, it is announced to the pilot and may be below his limits for landing.

Either way, it is not that people here 'don't know'. Without being rude, your question is totally impractical, but you seemed unable to accept that. If a skid is detected on one side, the anti-lock brakes will release the brakes on the other side, so there is no basis for your question. It is obvious you have zero experience!

[runway2]

oh ok, i honestly thought it was an issue, cos in an earlier Prrune forum one person reported this issue when they were discussing about runway width.
PPRuNe Forums - runway width requirements
i guess it is not the case,

can anyone help with suggesting the maximum possible turning rate for an aircraft? if there is one.

thanks

[BOAC]

the anti-lock brakes will release the brakes on the other side,

- are you sure of that.................?

[TyroPicard]

There is always a small piece of gold at the end of Rainboe's posts.....

[BOAC]

gold??????????????

[runway2]

oh ok,, didnt knw this was a trivial issue,
i was just refering to an earlier,, prunne thread where a person by name of overun said that in differential friction was also and operation issue..

[BOAC]

runway - 'differential friction' - an anecdote for you:

Location: RAF Leuchars in Scotland, Runway 09
Year: 1971, January
Time: 0300
Boac returns from a 5 hour flight in a BAC Lightning, having said 'hello' to some Russian friends up near Iceland for a few hours. 'Runway and taxyways clear of snow and ice'. Deploy chute, then jettison same and trundle towards the end of the runway. At the end at about 20kts I start to turn left across the 'Operational Readiness Platform' (ORP) to take the taxyway off for a pee and a coffee.

Unknown to me (and not at my sharpest at 0300), the ORP had NOT been deiced and was sheet ice. Left main and noswewheel now on the icerink. Start braking to make the 90 off - oh oh....a/c starts to turn right at the right brake does what it is designed to do on a good surface. Ah ah! Try noswheel steering - nope - not interested. By now the right gear is on the ice and I have this element of rotation to starboard, like a second rate Torvill and Dean. After a complete 360 on the ice, seeing the Bell Rock lighthouse, St Andrews, Guardbridge and Leuchars lights pass across my vision, more by luck than anything I stopped, pointing in the right direction. Hmm - engines off, handbrake on and call for AA Relay to take me home.

No - I hadn't given a moment's thought to working out any yaw angles.

To answer your question - no-one knows. With main gear behind the c of g an aircraft should straighten up from a yawing touchdown. All you need is sufficient runway width to accommodate the antics.

[john_tullamarine]

a person by name of overun said that in differential friction was also and operation issue

For background, OverRun is an highly experienced airports design engineer but not an aircraft operator, per se.

If you want techo information on airport/runway design and construction, he is our resident expert and his advice should be given appropriate weight. His comment regarding differential friction is perfectly reasonable and relates to the engineering/Standards desire to have reasonably uniform frictional characteristics across the runway .. consider the analogous situation in your motor vehicle on a very wet road at speed .. each time you put one side of the wheel geometry into a puddle, the car suffers a quite noticeable yawing/steering torque to that side. Alternatively, under braking, one can see some lateral to-and-fro motion as frictional characteristics vary at each wheel location - although ABS tends to mask the effect albeit at the expense of a noisy ride.

BOAC's Lightning anecdote, likewise, is illustrative.

The main aircraft consideration for runway width is landing gear geometry per ICAO recommendations which, as far as I am aware, makes no concession to handling realities.

A subsidiary consideration arose quite some years ago wherein a recommendation was made to incorporate worst case handling testing on a Type basis to check that the geometric basis for matching Type to width wasn't too far wide of the mark (so to speak).

So far as I am aware, only Australia took that recommendation up and actually did some testing and I was involved in testing several Types. Some of the preliminary simulator studies done by the Australian Regulator's flight test section came up with interesting data which suggested that the geometric model was quite flawed for some Types.

I'll send a note to OverRun .. perhaps he might come into this discussion and offer some tech design comments.

So far as the OP is concerned, I think that the poster may have applied an incorrect emphasis to OverRun's comments elsewhere ?

.. and, for runway2, I'm afraid that you will just have to maintain good humour in the face of the occasional good-natured banter arising in threads such as this ...

[runway2]

thanks, pls do let me know if u have any documents wrt to the aussie study
i am just looking at runway pavement performance, so need to brush up on my aircraft operational aspects..
cheers!

[OverRun]

runway2,

a person by name of overun said that in differential friction was also and operation issue

john-tullamarine has gone to the heart of the answer in terms of separating the operating-piloting issues, and the airport engineering issues

. . the engineering/Standards desire to have reasonably uniform frictional characteristics across the runway .. consider the analogous situation in your motor vehicle on a very wet road at speed .. each time you put one side of the wheel geometry into a puddle, the car suffers a quite noticeable yawing/steering torque to that side.

Yaw angle and the like are operating-aircraft issues. Differential friction is predominately an airport engineering issue.

I have wondered from the start of this topic whether the question was some sort of indirect airport engineering question which is testing the acceptability of a runway deficiency – maybe a rutting/ponding problem which already exists or maybe someone is thinking of grooving the runway only 30m wide on a 45m wide runway or maybe the runway is widened (or is going to be widened say to 60m for the A380) and the surface of the widening is different to the surface of the existing runway?

Well the answer to that is short and simple. Deficiencies in rutting/ponding or friction are not acceptable.

The earlier PPRUNE discussion that you referred to was about minimum runway width, and the conclusions reached related to Vmcg and engine failure on takeoff. That discussion should not be extrapolated and applied to the issue of differential friction. Differential friction as a specific airport engineering issue would have to be addressed using other means. This is a very specialised area and only a few names like AEA or NLR come to mind as being able to do this.

[HarryMann]

just wanted to find out under what scenario an aircraft would veer off a runway due to differential friction/ i guess u guys do not know that

This would be something (I would have thought) that would be evaluated (or would have, years ago) by the a/c manufacturer and certification authorities, in general terms. Based on a broad spectrum of analysis and tests, limits, issues and a general understanding would have been developed. This would be carried over to new a/c design, using scaling/modelling of some sort - 'if' it was likely to be an issue.

It could well be that, due to mass and inertia considerations, it is a 'least significant' issue until the a/c has slowed considerably.

There are (at least) two distinct phases during landing, as far as kinematic antics are concerned - above and below the speeds at which aerodynamic forces predominate over runway frictional forces. Although as ever, there are many grey areas no doubt... where combinations can have profound influence.

In short, there are probably few if any 'strict' criteria on acceptable runway yaw angles, and if there are, it would be more likely structural load considerations than 'controllability' issues that were at the root of any such figures.

You can thus hopefully now accept, that specifying a single kinematic criteria when probably a dozen are involved, is considered a little naive especially when this sort of behaviour is way and above what a pilot is concerned with - if the show gets well out of line at some point, all one can normally do is use what aerodynamic, steering and braking corrections are available at the speed and conditions one finds oneself in.

[runway2]

Thanks for the info, appreciate it!