Actual Landing Distance WET - how is it determined?

[Charly]

I have a question:
Actual Landing Distance dry:
50 ft, VREF, dry RWY, Max manual braking, spoilers, no reverse
Required Landing Distance dry:
the above x 1,67
Required landing Distance wet:
Required LD x 1,15
Clear so far....
What is it with the Actual Landing Distance wet (or even contaminated)?
Has it been demonstrated similar to the dry case (Flight testing)?
Or has it been calculated on the basis of ALD dry?
The "search" function didn't help, and a qiuck search on my OMs didn't help either...
Thank you and best Regards
Charly

[Mad (Flt) Scientist]

Considering FARs (and bear in mind there are differences here both in terms of between authorities and historically also):
§121.195 "Airplanes: Turbine engine powered: Landing limitations: Destination airports" pretty simply lays out that the required landing distance for "wet or slippery" conditions is based upon a simple ratio of the required dry distance, per your summary [RLD(wet)=1.15*RLD(dry)=1.15*1.67*ALD(dry) ] UNLESS the manufacturer has approved data which is different (with the caveat that RLD(wet) may never be less than RLD(dry) )

There is therefore no requirement on the operators to have access to "real" ALD(wet) data nor any requirement on the OEM (in Part 25, say) to produce such data for the operators' use. This means that there's no incentive for either to have that data specifically available unless it's better than the 1.15 factor would be (unless they care about people sliding off runways, of course). Even if it were available, it wouldn't be "approved" data - because there's basically no reg to approve it to - and would end up in a non-approved supplement to the flight manuals.

IF specific data are provided they'll have been generated based on a combination of flight test data and empirical data for the effects of various states of the runway (and the same goes for contaminated data, for snow, slush, etc.) but if no specific data for the ALDs are given then the RLD(wet) is simply defined by rule from the dry performance and may not relate to the real wet performance of the aircraft.

[Paris Hilton]

ACTUAL landing distance (wet)?

Observe touchdown point and stop point. Get tape measure and measure distance between points.

Sorry, couldn't resist.

[Charly]

Thank you scientest.
Reason for my question was this: Airbus has provided us with "Actual Landing Distance WET" and even Contaminated (standing water, slush, snow, ice) to calculate actual landing distances for failed aircraft equipment (see also http://www.pprune.org/forums/showthread.php?t=271027 ). And when I'm not mistaken, JAR requires an aircraft with system failures to meet actual landing distances only.

Regards Charly

[Mäx Reverse]

As far as I'm informed, only the DRY case is flight tested, i.e. demonstrated. WET and CONT are only calculated.

Regards MAX reverse

[Charly]

Danke Mäx.

[Mad (Flt) Scientist]

Not quite; although, for example, a full test programme won't be conducted for wet runways (its hard to get exact runway states anyway, so there'd be some scatter in the data collected) - but the anti-skid system will at least be assessed in non-dry conditions, and any data gathered used in calculating the performance on wet runways.

(Even the dry data is "calculated", since the demonstrations are no longer done as one single performance landing, but in a piecewise fashion; a methodology encouraged after Douglas got a bit too enthusiastic during a landing test, the video of which is widely circulated...)

[john_tullamarine]

And when I'm not mistaken, JAR requires an aircraft with system failures to meet actual landing distances only.

If your comment infers that you see no problem with unfactored data that frightens the daylights out of me. Having seen some performance landing tests in years past (and some of the aerodynamics fudges) I wouldn't count on stopping in the suggested raw distances in a fit .. ie the data is better used as the brick wall minimum distance .. and then one should review all reasonably available runway options .. with the aim of trying to push up the distance factor from 1.0 to somewhere closer to 1.67 x 1.15.

If you don't have that luxury for some operational reason .. then at least you have an idea as to how critical or relaxed the proposed landing operation might turn out to be ... and base your real time howgozit assessment on that benchmark ..

[Charly]

If your comment infers that you see no problem with unfactored data that frightens the daylights out of me.

I don't understand how you got the idea, I wouldn't see a problem??

If JAR requires only Actual landing distances for failures and Airbus supplies me with this data, the next question is (out of interest), how those are actualy determined for wet or contaminated runways.

The definitions I quoted in my first post concerning actual landing distance should have shown you, that I am certainly aware of the nonreproducability of these landing distances. (Vref, 50 ft, max manual braking ...).

[alf5071h]

JT, I agree with your concerns.

If anyone can help, I would appreciate chapter and verse, or at least an explanation with assumptions for any JAR use of actual (non-factored) distances.
Beware the terminology and biases of language; there are many pitfalls in the regulations. N.B. recent FAA pronouncements on contaminated runway operations.

Charly et al, “…how those are actually determined for wet or contaminated runways.”
As MFS explains manufacturers do use some measured data to validate the calculations; yes I have measured wheel marks in the snow!

Some insight to the calculation of JAR contaminated landing distance is in NPA 14/2004 ( http://www.easa.eu.int/doc/Rulemaking/NPA/NPA_14_2004.pdf ). Although this material is not yet in the regulations, most manufacturers use this basis or similar analysis for their published data.
Note some significant items:-
“Due to the nature of naturally occurring runway contaminants (standing water, slush, snow, ice) and difficulties associated with measuring aeroplane performance on such surfaces, any data that is either calculated or measured is subject to limitations with regard to validity. Consequently the extent of applicability should be clearly stated”.
Thus, an important aspect when using data relating to these conditions is to understand the assumptions and limitations in its use. Not all manufacturers publish these, or those that do, the notes are often in the ‘small print’, i.e. what is a ‘Boeing slippery runway’. (http://uk.geocities.com/[email protected]/alf5071h.htm) See the small print on slides 12 & 13. Also see Stopping on Slippery Rwys (Boeing). (www.pilots.or.kr/upfile/aip/9AAAA_StoppingonSlipperyRwys2.ppt)

NPA 14 continues:- “Standard Assumptions. Due to the wide variation in possible conditions when operating on contaminated runways and the limitations inherent in representing the effects of these conditions analytically, it is not possible to produce performance data that will precisely correlate with each specific operation on a contaminated surface”
Most data assumes that the point of touchdown is 1000ft in from the threshold; I suggest that few operators achieve that in normal conditions so why expect this accuracy when the conditions may not be ideal, worse still with systems failures etc. Then there is threshold crossing altitude, braking techniques, use of reverse, etc; see Managing the Threats and Errors during Approach and Landing. (www.flightsafety.org/ppt/managing_threat.ppt) for further insight.
Another consideration should be the safety ‘factor’; remember that even where 1.67, 1.92, etc is used, aircraft still continue overrun the runway. A Canadian study (on snow/ice) concluded that a factor of 2.2 or 2.4 might be required in order to provide the equivalent level of safety as on a dry runway; this implies that as the factors increase – the runways become wet or contaminated, the operating risks increase disproportionately and the crew are expected to change their procedures (behavior) to make up for this shortfall in safety.

Having once visited the end of a limiting wet runway using the wet-factored distance for the configuration - flapless, good T/D position, speed, braking, etc, I would not recommend the use of unfactored distance as a stopping distance. For those extremely rare situations of having to land on an ‘unsuitable’ runway, the plan (approach briefing) should include “in the event of an overrun” actions. The specific issue in my landing was that wet, ungrooved concrete has a very low coefficient of friction (with respect to tarmac), where the manufacturer’s data was ‘at the limits of its applicability’ – as were the crew …. … !

[john_tullamarine]

Having once visited the end of a limiting wet runway using the wet-factored distance for the configuration

.. oh dear ... and, no doubt, followed up that evening with a calming scotch or three .. ?

The JT/ALF joint prosecution rests ..

[Charly]

Look guys... it's not about stopping the aircraft at the value out of the table.
I don't want to smash the aircraft onto the 1000 ft marker with VREF Speed, already with the brakes pressed in the air and pulling the spoilers 2 ft above the rwy. It's not about that!!

Following example:
You have lost two hydraulic systems, and the longest Rwy in your vicinity is 3300 m long. Can you land or not:

An A320 with 62 to for Landing:
Actual Landing Distance Dry : 900 m
Required Landing Distance WET ergo x 1,67 x 1,15: 1730 m

Actual Landing Distance WET (from my handbook): 1200 m

Correction factor for a dual Hydraulic Leak x 2,6 (normally applied to actual distances): 4490 m required or 3120 m actual.

So, gentleman, would you land on a 3300 m RWY?

[mutt]

Yes.....

1200m (wet distance book value) x 2.6 (Hyd Fail) = 3120 meters

The 1.15 conversion is a MINIMUM value, actual calculations may increase this, in your example DRY=900 * 1.15= 1035 meters, but Airbus testing resulted in a value of 1200 meters.

Mutt

[Mäx Reverse]

To questions in the first place:

1) Which Hyd-Failure? G+Y? G+B? B+Y? The Ldg Dist Factor varies significantly....

2) Does your company already use the revised Calculation Procedure (we introduced it SEP06). Obvously not, I also foung the 2,6 for a G+Y fault in the 'old' calc procedure.

From my QRH:

A320, 62 ton, Sea-Level, 0 Wind, WET Rwy: 1.150 Meters minus 6% for 2 Reversers operative = 1.080 m

G+Y HYD Fault on Wet Rwy: 2,45 x 1.080 m = 2.650 m
G+B HYD Fault on Wet Rwy: 2,00 x 1.080 m = 2.160 m
B+Y HYD Fault on Wet Rwy: 1.90 x 1.080 m = 2.060 m

Maximum Reverse is considered on all operating engines on a wet Rwy, thus the higher coefficient for the G+Y fault.

Given the worst case of G+Y, I think 3.300 meters is plenty of a margin.

Calculating it with the 'old procedure' and assuming you need 3.120 m and have 3.300 m available, it's gonna be very tight. If there are no other airports available in the vicinity, you'll have to do it, as there's 'LAND ASAP' on the SD and if the tiny B ELEC Pump quits, the you look a lot worse than after leaving the RWY END at 40 knots.

Good Luck, MAX ;-)

[Charly]

Hi Mäx, we have the new values as well already. This was still an old example from an OM-B copy on my laptop (private Partition ;-) ). It wasn't concerning the exact values anyhow.
Take a 2800 m RWY for the new values to exagerate the dilemma of not being able to consider required distances with the "abnormal factor".

[wee one]

This has been done to death before.Dont do airbus but in relation to boeing and jar
There is no need for distances to be factored after the performance dispatch phase. Non normal landing distances are issued as seprate data and allow for the failure.(i refer to boeing PI tables) This is all contained in the Performance in Flight section of the qrh. Wet distances are found using Slippy runway braking action good.

The distances derived are doable, not black magic. As a regular operator into short airfields i can confirm. Our dispatch numbers are limiting but eg the perf brief for JSI has a caveat that once dispatched there is nothing stopping the Commander using the PI for actual conditions and config should a sudden rain shower occur for example after dispatch based on the weather, alternates etc. That means If the wx is fcst ok and one of the alternates is doable with wet dispatch lengths (ldrx1.67x1.15) then the flight can go and the commander asses the destination in real time with real distances.he cannot dispatch with more than the moast limiting Factored wgt for conditions but can do the above

Places like a suddenly wet Corfu rwy 17 would recieve much business if it wasnt the case.1900 lda and usually aprox 1700 wet ldr 737/8 as derived from the pi with max ldg. (memory only not exact)It does recive the business and people dont dissapear after factoring un necessarily
Just to reiterate..There is no requirement for factoring after the dispatch phase under Jar 1.51. The commander must satify himself sufficient distance exists. There may be a company policy of factoring, thats different, lesys not years of a certian operation cloud the legal facts. JT you are entitled to factor as you please but your initial tone was a tad condesecnding as if to point out that lesser mortals have got the reins and are daring to fly the numbers which you find disturbing. Fact The numbers work notwithstanding your personnal experinces or company polcy.

I aplogise if the reply is disjointed I'm trying to multitask

[john_tullamarine]

Wee One,

Apologies if I appeared condescending .. that certainly was not my intent.

There is no need for distances to be factored after the performance dispatch phase

True. However, I suggest that, on the line, one is better served by using the QRH data as being questionably achievable, absolute max effort numbers. I have my engineering reasons from previous lives upon which to base such a comment ..

That is to say, use the data as one more bit of input into the decision process regarding where you might be intending to recover the aircraft in an abnormal or emergency situation.

The distances derived are doable, not black magic

... a point which could be discussed .. upon what basis, and with what considerations, do you so assert ?

There is no requirement for factoring after the dispatch phase

Concur .. but that is not the point .. the QRH data ought to be used as decision making input, not gospel. The emphasis ought to be on checking what factor might exist - which then gives an indication of what level of adrenaline/care is called for - rather than imposing a specific factor ...

[wee one]

JT ,
It seems we are in agrement on some points. I was merely pointing out (probably in a endless procrastinating way...ooops doing it again) the difference between what may be considered prudent and what is legally required. From my personnal experience it is achievable and reliable data. This includes regular ops into very limiting strips and many contaminated winter fields.

I feel these days there is alot of hand me down info circulating that is a hybrid of Regulation and mixed sops all churned up in the hanger talk sessions and surfaces as fact or Regulation. As a result I like to stick to the actual facts first and then discuss the merits or demerits of applying extra for granny on any calculation be it perf or fuel carried for example.

If a Commander chooses to ad distance or fuel or anything more than needed then it is good to know the baseline from which to start. But not to confuse personnal preference with the base line, if you know what I mean....I'm not sure I do:8.

There is also the occasional time that many guys will hide behind safety etc when really it could be just inexperience in a particular kind of op. At the tea and buscuits afterwards it would be prudent to have your decision based on fact not a hybrid version. EG Pilot one on first fight on cat c ops diverts due to factoring PI. Pilot 2 with more experince lands safely in a distance well within the PI non factored length. Pilot 1 thinks pilot 2 is a cowboy. At the tea and buscuits it tanspires that Pilot 2 was correct and pilot 1 had based his decision on thinking he had to factor and was finding answers that suited his nervous confirmation bias..

Enough ...Ive been procrastinating again

[wee one]

JT,
The short answer...I mostly agree. See above

[mutt]

When deciding on company policy we look at what is achievable by an average pilot, we therefore do not consider the "actual landing" distances as usable unless they are factored or its an emergency.

Maybe we are just unique :)


Mutt

[wee one]

Ah thats the difference. We dont have any average pilots:}

Sorry couldnt resist that one:ok:

[Mäx Reverse]

When deciding on company policy we look at what is achievable by an average pilot, we therefore do not consider the "actual landing" distances as usable unless they are factored or its an emergency.

Maybe we are just unique


No, you aren't... I enjoy the luxury of working for a company who isn't operating to legal minima in most areas.

In this context actual landing distances are only used in case of landing with technical abnormals or emergencies. For dispatching, operating and even diverting in flight Required Distances are considered.

As out Flight Safety Department is just starting a major campaign 'Avoiding ALAs' I recently saw some very interesting statistics from our Flight Data Monitoring, where Required Landings Distances were compared to the distances achieved in our daily ops. Operating in 'limiting' airfields, maybe under adverse conditions, using Actual LDs sounds like an accident waiting to happen. At least with an FBW-Airbus.

I drove the BAe-RJ85 before, so I guess I can say I saw a lot of landings into 'limiting' airports like LCY or FLR. However I personally rate the braking capability of the RJ (which even hasn't Reverse) much better than the A319-A320. AZ (or was it Meridiana) operating with A319s into FLR, wouldn't do it:ouch:

Still by talking to colleagues of other airlines who are operating to JAR-OPS+0, I wonder quite often how many ways JAR-OPS offers to bring yourself into trouble perfeclty legal.

Nothing beats success - What's the other side of the medail?

Regards, MAX

[alf5071h]

It may be just one of those days, (multitasking or I’m just below average), but I don’t follow all of the reasoning in accepting a 3120m actual landing distance on a 3300m runway without explanation of the assumption. As discussed extensively above, the industry accepts a 1.97 ‘factor’ (1.67x1.15) on top of the actual dry landing distance. ‘Accepts’ is used in the sense that this does not necessarily provide the same margin of safety when landing on a dry runway with a factor of 1.67, but it is equivalent (“equal, all things considered”) i.e. satisfactory for a range of normal operations provided the relevant ‘things’ are considered.
So why should we accept a significant reduction in safety margin in the use of 3120m as opposed to the fully factored distance? Is this because whenever there is a failure the normal level of safety is reduced?
I calculate that for the Hyd failure case the fully factored distance is approx 5590m, a significant increase compared to 3300m available - I am not suggest that 5590 is a ‘must have’ distance, it’s for comparison. So what are the justifications for accepting a low ‘safety factor’ of approx 1.06 (3300/3120) vs the standard factor of 1.97 (wet factored failure case 5590/3120).

In exceptional circumstances the Captain has the authority to accept additional risk, i.e. lower the level of safety. Is the hydraulic failure a qualifying event? This depends on many other aspects of the situation, particularly the availability of a suitable runway. Is this what Airbus state, imply, or leave open to the operators interpretation in calculating risk (and bearing the responsibility). In the failure case, the question to be considered is ‘should we be making this landing’, not ‘can we make this landing’.

However, confining the discussion to 3300m available and using the actual landing distance as the basis (Decision made - yes we should be making this landing, even after considering the distances involved, because there is no other alternative runway), then the calculations above should be used to quantify the change in risk – the level of safety of the planned operation. Thus equally important are the changes to procedures, operating conditions, specific actions, etc, based on the additional risk and any particular characteristics of the runway/airport in this situation, i.e. this is not just a landing distance question it is one of risk management.

I base my comments on the UK CAA AIC 14/2006 ‘Landing performance of large transport aeroplanes.’ I quote -
“2.3 This gross distance (actual) should be regarded as a theoretical minimum, consistent achievement of which requires a high level of pilot skill under favourable conditions, and concluding in a level of deceleration that would normally be considered excessive from the passenger comfort point of view. Consequently, in order to provide an operationally realistic level of performance, this gross distance is multiplied by a field length factor to obtain the Landing Distance Required.
2.4 This factor accounts for the normal operational variability that can be expected in day to day service such that the chances of a landing overrun are remote. It should be appreciated that the value of the factor is not the same for all aeroplane types. For example, propeller aeroplanes are not as sensitive to excessive approach speeds as are jet aeroplanes, consequently they are subject to a smaller factor.”

“6.2 Where a failure occurs in flight to other systems (eg restricted use of high lift devices) crews will normally be provided with advice on their effect on landing distance. Allowing for the low probability of such an occurrence, lower safety factors, if any, may have been applied compared to those used for the normal conditions. Therefore, even when such performance corrections are taken into account there may be a reduction in safety margins available to account for variations in, for example, touchdown accuracy or stopping technique. Other conditions, such as reduced wind limits, may also apply in such cases.”

So for a 3300m runway using a relative safety factor of 1.06 / 1.97 as a guide to the risk involved, and also against the need for “a high level of pilot skill under favourable conditions”; then do you land on 3300m?
If yes, what additional restrictions and guidance would be given? If operational managers cannot be assured that every pilot will be able to provide the high level of skill required on that day or in those particular circumstances (wee-one, you cannot determine “P1 non-factored landing distance’), then are they prepared to mitigate the consequences of landing (or overrun), because they, like the crew may have no other options when deciding to land.

Avoid wet runways?

[alf5071h]

Mutt, where does the minimum ‘conversion’ of 1.15 come from? You appear to have used it as a means of determining the actual wet landing distance, although you note that the book value is larger - possibly because they are not related by a simple conversion.
My understanding (as above) is that for operational certification, a ‘field length factor’ is applied to actual distance to provide an additional level of safety - the required landing distance. An additional distance equal to 15% (1.15) of the factored dry distance should be added when operating on wet runways. This is a minimum distance for operational certification, but operators may increase this according to circumstance. Have I misunderstood something here?
Applying multipliers to aircraft landing distances is not the same as adding field length safety factors; the latter are an attempt to maintain a level of safety and not necessarily about where the aircraft will stop, although in practice they may be a good guide.
Were you relating 1.15 to the recent FAA guidance material? I found it confusing; I would appreciate a pilot’s guide to what it means or might mean in relationship to the discussion in this thread.

[wee one]

Alf I typed PI (eye) not P1. Performance in flight in boeing qrh.

The distances derived are unfactored. Or they have wasted ink typing that specific phrase. Jar 1.51 does not REQUIRE any further factoring, when using these charts to determine landing distance DURING a flight.. These are actual distances achievable. The non normal charts have different distances for different problems. The emergy effect is allowed for but they are still unfactored vis a vis LDR.

I have tried to make two points.
1 There is no requirement to factor under jar after the flight is dipatched.Factoring is a function of perf A probablity, not actual on the day.
2 Factoring of PI data is an operational option not a requirement.

To assume that operators that choose to use actual data in these circumstances are dangerous is unfair. The data doesnt support that and the achievable results on line and in the sim demonstrate that factoring is an academic safety proceedure to achieve the probabilty that perf a is based on.:ok:

If you factored the ldr for an urgent heavy 763 problem with no fuel dump you would be looking a long time for a runway long enough to land.
An average long runway in this case being 3000m would require a PI Base distance of approx 1550 meteres. So lets say the PI unfactored distance (from the PI non normal charts) was 1900 metres and its wet. LDR then becomes 1900x1.67x1.15=3648m ldr.
It starts to get silly distances very quickly. Mlud the defence rests:ok:

[mutt]

Alf5071h,

Sorry the word "conversion" is incorrect in this context, i should have used "multiplier".

Mutt

[Charly]

this is not just a landing distance question it is one of risk management.

Everything(!) in aviation is risk management! Stepping into an aircraft deciding to fly is risk management.

I'll always be happy to land on runways that fulfill the JAR required distance (my operator requires it that way in all cases except emergency).

When I have a heavy technical problem, or I need to divert immediatly (smoke), I will choose the next airports in my immediate range, choose the best runway for my needs (length, width, state, crosswind, type of approach etc...) and will land there.
And in some cases I'll have to land with actual landing distances with only little safety margin (for example + 8% for 5kt high on Vapp).

[zon3]

JAR–OPS 1.475 General
(a) An operator shall ensure that the mass of
the aeroplane:
(1) At the start of the take-off;
or, in the event of in-flight replanning
(2) At the point from which the revised
operational flight plan applies,
is not greater than the mass at which the
requirements of the appropriate Subpart can be
complied with for the flight to be undertaken,...etc.

Does this not mean that in the event of an in-flight replanning, same factors apply as during planning phase/dispatch?
If you have the answer, please let me know where it can be found as well.

z3

[Mad (Flt) Scientist]

I would say "yes", an inentional replanning BY CHOICE would be subject to the same restrictions as pre-flight planning. (Anything else would be ridiculous, as you could otherwise routinely "replan" 1 minute after takeoff and legally ignore every rule in the book!)

But that requirement wouldn't be interpreted to apply to abnormal or emergency conditions. Even if it DID, there's a "notwithstanding clause" in JAR-OPS which says, in effect, that you can break any rules you like in an emergency, anyway.

[zon3]

I totally agree with you, Scientist. Re-planning by choice (company's choice?) is a completely different situation from the "emergency" case, and (based on JAR-OPS) it seems obvious that the same factors should apply - why reduce the safety margins during normal operations just because you're airborne?


Other thoughts or opinions, anyone?

[alf5071h]

“Does this not mean that in the event of an in-flight replanning, same factors apply as during planning phase/dispatch?”

Yes. JAR-OPS 1.515 requires a landing distance factor without reference to the aircraft being before or after the ‘start of the take-off’. So while 1.475 ensures that the aircraft is within all of the mass limitations for flight and the planned landing, 1.515 requires a landing factor for the planned landing and in-flight diversion. Abnormal or emergency operations would be considered on ‘merit’ as discussed in this thread. Note that IEM 515(c) requires the before takeoff plan to be based on a lower landing mass if ‘other considerations’ apply i.e. longest runway not available due to ATC, noise restrictions, wind, etc.

Re stopping with double Hyd failure: I assume that steering could be limited, thus asymmetric reverse might be used (if available) when the rudder effectiveness decreases (even if that is available with the failure). Using reverse for directional control may increase the ‘actual’ landing distance. Also, the amount of directional control could be limited in a crosswind (particularly on a wet runway), so there may be additional limits in those conditions i.e. landing distance could vary with crosswind.
Is there any manufacturer’s advice on these issues, or is this also the crew’s responsibility to manage the risks? The management process has to take into account all considerations (and consequences) of rare system failures; the manufacturer may only provide a stopping distance as it would impracticable to cover all foreseeable circumstances.

[Mad (Flt) Scientist]

Where a failure case covered by a single flight manual procedure has handling difficulties or special procedures, these will be noted in the relevant procedure (such as advice to minimize brake pumping in the event of multiple hyd failures, if your brake system is running on accumulator power only).

A double or multiple failure which is not the subject of a specific AFM abnormal or emergency procedure may have handling or other problems due to the combined effects; these may not be identified, since there won't be a "engine fire plus depressurisation" procedure to follow, and the problem may ONLY exist in the combined case. In that case it is indeed on the shoulders of the crew's basic airmanship skills.

The idea is that such 'perfect storm' days are so rare that they should not be provided for, as there are such a huge number of combinations, each of which is very unlikely indeed.

PS obviously, those remarks apply specifically to our manuals, but I don't believe we're significantly different from other OEMs.

Edit: to clarify - usually double hyd failure is a sufficiently probable event that a specific procedure exists, which would include some or all of the advice suggested, dependent upon model.

[tribo]

Video of Boeing 787 Dreamliner wet runway test in Roswell, N.M. at this link

Video of Boeing 787 Dreamliner wet-runway tests in N.M. (http://blog.seattlepi.com/aerospace/archives/218687.asp)