Do aircraft manufacturers assume damp runway is wet?

[grimrod]

In the past it has been assumed that a damp runway is considered to be dry. This is still the case under EU Ops.

It is now reconised that in fact a damp runway as commonly defined does not exhibit the same braking characteristuics as a dry runway and hence treating a damp runway as a dry runway may no longer be appropriate.

What do aircraft manufacturers assume in their Flight Manuals? The traditional manual only has dry and wet data. What do manufacturers assume is a damp runway? Is it wet no matter what EU Ops or any other Operational regs state or are they sticking to damp=dry??

[Capn Bloggs]

Damp + Grooves = Dry?

[safetypee]

Essentially manufacturers only have to consider a dry runway, this is the basis of factored certificated performance in the flight manual. Wet performance is factored dry.

Using dry performance on a damp runway is an operational issue which is in EU-OPS 1.480(a)(4); “… those paved runways which have been specially prepared with grooves or porous pavement and maintained to retain “effectively dry” braking action even when moisture is present.”

The UK CAA warned operators of the importance of using the correct performance in FODCOM 2009/03
“It is not sufficient for a runway to be considered, for performance purposes, as dry when it is wet solely on the basis that it is constructed with, for example, grooves or porous friction course pavement. Dry runway performance must only be used when the CAA has accepted in writing that the aeroplane can actually achieve the “effectively dry” braking action referred to in the EU-OPS definition.
…, there is currently no provision in the UK for notifying operators of runways having such surfaces, and the CAA is not aware of runways elsewhere that fully meet the “effectively dry” criterion.” (My bold)

The FODCOM has been superseded by CAP 789 ‘Requirements and guidance material for operators’, para 5.4.

“ a) Although a runway may have a grooved or porous surface, it may not be possible to demonstrate that it retains an 'effectively dry' braking action when wet. This may be because the type of surface is inherently not physically capable of retaining dry braking friction characteristics in the presence of sufficient moisture to be termed 'wet'. In other cases, it may be as a result of a surface reaching the end of its design life or that the recommended routine maintenance procedures have not been sufficiently effective … This is particularly relevant in the case of accumulation of rubber deposits in touchdown zones.

b) The effectiveness of aircraft anti-skid systems is especially sensitive to the presence of water on the runway. Aerodrome operators' Continuous Friction Measuring Equipment does not record a corresponding response to the presence of water, and thus operators may not rely on runway friction reports alone for demonstrating that a runway can retain 'effectively dry' braking action. Verification of the aircraft's braking performance capability in wet conditions is also required.
This would normally require support from the aircraft manufacturer.”

i.e. operators may not know or be able to verify how damp the runway surface is, nor quantify the maintenance condition of the runway; thus in order to plan for a safe landing, damp = wet.

[AerocatS2A]

safetypee, your post doesn't mention damp runways at all, only wet grooved runways.

The AFM for that most modern of passenger jets, the BAe146, defines damp as being neither wet nor dry but as being equivalent to dry for performance purposes. A wet runway has enough water to appear reflective but no significant standing water.

As for grooves, not having seen any concrete information on how to treat a grooved runway, I'll take grooves as giving me better than expected braking performance on a wet runway, but I wouldn't use grooves to redefine a wet runway as dry.

For landing performance I'd consider a damp runway wet because I'm not able to assess it myself and I might not have the option of holding if the so called "damp" runway turns out to be wet.

[HazelNuts39]

Do aircraft manufacturers assume damp runway is wet?It is not up to the aircraft manufacturer to make that assumption. The manufacturer AFM performance information is provided in accordance with the applicable airworthiness regulation, using the runway surface definitions specified therein.

[FLEXPWR]

Damp runway with significant rubber deposits will be much more slippery than a wet runway! Assumptions are always based on ideal conditions, but this can be easily overlooked when it comes to real life.

[safetypee]

AerocatS2A, re #4 :ok:

My belief was that previous advice on grooves related to damp vice wet, but I am unable to locate any earlier documentation relating to this or anything different in the old JAR-OPS.
A conclusion might be that this is another gap between aircraft certification and operational regulation, where the operator is left without advice other than to choose a safe option.

Re 146; I would be surprised if the quoted wording was in the AFM. In the Ops Manual may be, but then the data would be advisory.
I recall that one operator (possibly UK) operated on a very short grooved runway in Europe to the effectively dry requirements of JAR-OPS in wet conditions. I suspect that this was an operational approval and thus did not involve the manufacturer.

Be that I am surprised by the AFM revelation, then this would be an accepted position from both the manufacturer and the certificating authority; but even then the operator has to decide if the damp conditions reported are as the manufacturer intended/tested. The AFM damp definition would probably match the UK AIC (1.1.1), but then what about the rest of the world.

Thus in failing to answer the question and possibly adding to the regulatory confusion, we do appear to agree that damp = wet.

[safetypee]

In addition to the above, and hopefully without further confusion, perhaps the clue is in the EU-OPS wording / definitions.

EU-OPS 1.148
3. “Damp runway”. A runway is considered damp when the surface is not dry, but when the moisture on it does not give it a shiny appearance.
4. “Dry runway”. A dry runway is one which is neither wet nor contaminated, and includes those paved runways which have been specially prepared with grooves or porous pavement and maintained to retain “effectively dry” braking action even when moisture is present.
10. “Wet runway”. A runway is considered wet when the runway surface is covered with water, or equivalent, less than … … or when there is sufficient moisture on the runway surface to cause it to appear reflective, but without significant areas of standing water.

The definition of ‘Damp’ is not dry, but refers to moisture inferring two conditions – shiny / not shiny - damp / wet.
The ‘Dry’ grooved caveat also refers to moisture, but fails to delineate between damp (not shiny) and wet (shiny reflective surface). This leaves an ambiguous interpretation such that the dry grooved caveat can be either wet or damp. ???

Or has my understanding of plain English and logic failed me? :confused:

[HazelNuts39]

It is now reconised that in fact a damp runway as commonly defined does not exhibit the same braking characteristuics as a dry runwayDo you have a reference for that statement?

[FlightPathOBN]

HN39,

Check the RuFAB results...(Runway friction charecturistics measurement and aircraft braking)

RuFAB (https://docs.google.com/viewer?url=http://skybrary.aero/bookshelf/books/1504.pdf)

(ever used the SNOWTAM report???)

and

Runway surface condition assessment.. (http://www.iata.org/iata/RERR-toolkit/assets/Content/Contributing%20Reports/ICAO_Circular_on_Rwy_Surface_Condition_Assessment_Measuremen t_and_Reporting.pdf)

"4.41 Rainfall brings moisture to the runway, which will have an effect on aircraft performance. Flight test data shows that even small amounts of water may have a significant effect on aircraft performance, e.g. damp runways effectively reduce aircraft braking action below that of a clean and dry runway."

(based on ESDU data accounts that for variations in water depth from damp to flooded, runway surface texture within the defined texture levels, tire characteristics and experimental methods.)

[safetypee]

The following quote is taken from FAA document ‘Airplane Performance Harmonization Working Group, Task 1 – Airplane Performance Operating Limitations', circa Nov 1997.

The conclusion provided in the FAA Report for Project 308-3X typifies the results shown by the data in the other reports: “The absolute values of friction coefficient between the low reflective surface (damp) and high reflective surface (wet), where there were no large areas of measurable standing water, were approximately the same.” This conclusion is echoed in Engineering Sciences Data Unit Item Number 25, paragraph 5.2.2, which states, “In damp conditions, with the exception of surfaces such as I in Figure 7 [which is a surface with an open macro-texture and harsh micro-texture, such as a grooved or porous friction course surface], the coefficient of friction is noticeably reduced from the dry surface value, the effect becoming most marked on surfaces such as IV in Figure 7 [which is a closed macro-texture, smooth micro-texture surface].

i.e. Damp = Wet, and the grooved / porous exception is not declared as dry.

The document notes at that time that the JAA allowed a damp runway to be interpreted as dry, whereas the FAA (with minor exceptions) did not. It was suggested that the JAA should align JAR-OPS with the FAA position in order to harmonize the regulations.

[AerocatS2A]

safetypee, the words are indeed from the AFM. The actual quote is:


Dry runway

A dry runway is one which is neither wet nor contaminated. For performance purposes, a damp runway is considered to be dry.


It then defines a wet runway in much the same way as your EU-OPS quote.

The page was amended in 2007 so unless there's been a subsequent amendment that I don't have, those are the BAe words on a damp runway as approved by EASA.

Standard caveat: "legal" does not necessarily mean "safe".

[safetypee]

AerocatS2A, thanks, interesting. :hmm:
The AFM text appears to follow JAR-OPS 1.457(d) and thence with EU-OPS 1.475(d) which to my further surprise still states - “For performance purposes, a damp runway, other than a grass runway, may be considered to be dry”.
The 146 was a UK CAA / JAR aircraft, and it is quite possible that it included operational regulations. Thus the 146 was in accordance with JAR-OPS and remains so with EU-OPS.

Although BAe equate damp with dry, there is no guidance as to what additional risk is involved. Presumably the performance reduction from damp does not warrant the additional 1.15 ‘wet’ factor, but equally damp is not as good as ‘dry’. Operators are left to judge any additional risk and employ suitable mitigation.
The UK CAA (#3) may have attempted to clarify the situation, but without specifically including damp, the ‘damp’ ambiguity remains; i.e. EU-OPS damp may be considered dry – but not as good as dry, but wet grooved is not effectively dry (UK CAA); but is wet grooved as good as damp ?
EU-OPS states that damp is ‘effectively dry’;
FAA / NASA / ESDU research states that damp is not as good as dry;
the UK CAA state that wet grooved is not ‘effectively dry’;
some manufacturers state the operational requirements,
but no one states what the additional risks are and what mitigation might be assumed in the regulations.
So what happens if other operational regulations do not agree with EU-OPS (e.g FAR 121).
Would the originating AFM overrule FAR, or would the national authority remove the damp caveat in their version of the AFM ?

:confused: Thus full circle to the original question.

[HazelNuts39]

So what happens if other operational regulations do not agree with EU-OPS (e.g FAR 121). The more restrictive applies. because the operator must comply both with the operating limitations stated in the AFM and with the operating rules that apply to his operation.

P.S.
(1) The whole 'damp runway' issue strikes me as somewhat academic. When a runway dries up after a rain shower, it goes through a phase where it is not not really wet, nor quite dry, but could be considered to be 'damp'. Is there a special test before it is declared 'dry'? How often does an airplane have to wait for the runway to be declared dry, because its weight is such that can legally take off from that runway when dry, but not from the same runway when wet?

(2) "the UK CAA state that wet grooved is not ‘effectively dry’" As I read it, the UK CAA state that they do not have have the means that would enable them to certify that the maintenance of a particular runway will ensure that 'effectively dry' runway friction is maintained when wet.

(3) A simple definition of a wet runway: If you sit down on it and it feels wet, then it is wet.

[Max Angle]

Is it not the case that in EASA land (it is already being applied at BA) that damp is going to disappear and any runway that is not bone dry will be considered wet.

[safetypee]

See the notes in Takeoff and Landing on Wet and Slippery Runways (www.captainpilot.com/files/BOEING%20PERFORMANCE/Takeoff%20or%20Landing%20on%20Wet.pdf) which quotes data from testing published in NASA Technical Paper 2917.
The ‘effective friction’ is compared across dry, wet, and damp runway surfaces (page 25 - ).
The summary (page 31), states that over a range of damp surfaces the friction was reduced compared to dry, but may be better than wet; but that damp is a subjective term.
Data for grooved runways is also given (page 28).

[grimrod]

I tend to agree with you here.

I think we will end up with;

1. DRY
2. WET
3. CONTAMINATED.

If its not DRY or CONTAMINATED, then runway must be WET. Damp runways will not exist.

Seems the way its going.

[TheRobe]

It's critical as a pilot to understand the difference between wet, dry, contaminated, dusty, damp, moist, sticky, viscous, slippery and sloppy.

[grimrod]

Slippery runways and runways with a low coefficient of friction are another subject related to this for sure, which again brings in more questions about how a runway state is reported.

For the damp runway scenario, I guess what Im saying is that the airworthiness authorities are coming to the position that Damp=Wet and therefore if a runway isnt dry, its wet, and the need for a damp runway term is hence redundant.

[safetypee]

grimrod, #17 & #19, I agree.
The problems of reporting are to be addressed by the regulators via TALPA. An Airbus view of this is in Safety First (www.ukfsc.co.uk/files/Safety%20Briefings%20_%20Presentations/Airbus%20Safety%20First%20Mag%20-%20August%202010.pdf) (page 8 -), which indicates the use of standardised conditions and code to describe the runway, and thus provides a baseline for advisory landing distances.
Note that these require a damp runway to be considered wet (page 11)

TheRobe Re “It's critical for a pilot to understand the difference …” #18
Agreed, but what are these differences, and what understanding is required?
Dry, wet, and contaminated simplify the problem somewhat as there are specific definitions for these, and form the basis of certificated performance.

Dusty, damp, moist, etc, are sub-descriptions which probably should not be used, particularly as any effect on landing performance is not easily determined – other than generally adverse. They are not used in the TALPA matrix.
Boeing has used ‘slippery’ in a range of documents, some with differing definitions of a non-dry runway or an extremely wet / contaminated runway; yet their advisory performance categorisation only uses ‘good’, ‘medium’, and ‘poor’ braking action, but the ill-defined relationship between achievable aircraft performance and the runway condition creates opportunity for error.
TALPA refers to slippery (in parenthesis – code 3), but this is limited to describing a runway with less than the required standard of maintenance (slippery when wet).

So what do pilots really have to know and understand?

[TheRobe]

Safety -

In my operation we have Ref for a dry runway...damp reduces braking action so increasing braking distance roughly 5%, wet, roughly 10% or as dictated in the book...when the runway is sloppy, slippery, viscous, moist, or akin to a 16 year old girl having a rotten dream, we take the square root of the cubic inch capacity of the lav divided by nose tire pressure, then you multiply by the square root of the amount of cups of coffee, to get the new landing distance.