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11.2 is for contaminated runways and doesn’t mention a percentage. 11.1 is as I said above, 1.67 for dry and 1.92 for wet. The 1.92 is 15% more than the factored dry figure, not the actual dry figure. Edit: could be my copy is out of date of course... |
Sorry Bloggs and exfocx, I was working of an old version of the CAO. You are correct. |
Short when wet
After TriStar G-BBAI overran the wet runway at Leeds in 1985 - we all decided the Runway must be short when wet.
https://assets.publishing.service.go...987_G-BBAI.pdf |
From the very comprehensive and well considered report I could not identify the basis of the published landing data. Was the AAM based an ‘in house’ computation or did it use Boeing data - performance manual? Was the QRH based on Boeing ‘actual’ distances, or the more recent OLD method for calculating landing distance? Would the performance differ between documents, in addition to the likely difference between ‘actual’ and OLD? Do current documents still show numerical landing weight, where it is difficult to add 15% distance margin, or do they now show actual landing distance, even with the min 15% already added? General question: re “…15 per cent may be inadequate under certain wet runway conditions and subsequently encouraged operators to apply additional conservatism.” The FAA SAFO appears to precede the incorporation of TALPA recommendations and the publication of OLD/FOLD by manufacturers. Would the ‘inadequate’ comment still apply to FOLD calculations providing that the correct surface description is used? Notwithstanding the above, it does not degrade the value of the SAFO remark - need for ‘at least’ or ‘minimum’ of 15% safety factor, particularly as inaccuracies can arise from ground assessment, reporting, or crew interpretation of surface condition or reported braking action. |
Have a read of 20.7.1b section 11.2
20.7.1b is an operational document and has had a long history of the occasional discontinuity with the certification side of things. The AFMs know naught of the 20.7 series. 20.7 might be fine .. but it doesn't give you the data, necessarily, to comply with its requirements. |
JT, I as a lowly driver see it like this: 20.7.1bdetails the current regulatory requirements, including the "15% when airborne" rule. One can then use the AFM data (1.67) to work out if one can comply with the pretakeoff requirement (or one can use the AAM charts). Then, one could use the QRH data (1.15), when one is airborne, to work out if one can comply! :confused:
Originally Posted by Safetypee
Was the AAM based an ‘in house’ computation or did it use Boeing data - performance manual?
Originally Posted by Safetypee
Do current documents still show numerical landing weight, where it is difficult to add 15% distance margin, or do they now show actual landing distance, even with the min 15% already added?
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Does the old formula of 9 X the Square Root of the tyre pressure still get used for a rule of thumb? Given a tyre pressure of 180psi the aquaplaning speed would be approx. 120kts, 210psi would be approx. 130kts i.e. speeds to avoid where possible and unfortunately close to Vref at times.
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Hi parabellum,
Your formula is correct for dynamic aquaplaning when water can't be dispersed fast enough by the tyre tread - but the incident at Christ Church was due to Viscous Hydroplaning (mentioned in the final report). There is some good info at https://www.pprune.org/tech-log/1708...roplaning.html "Viscous Hydroplaning This one can fool you, because it can happen even at very low speed. Essentially, it is sliding on some liquid other than water or in a situation where water has mixed with something. For example, an area of a ramp or runway could become contaminated with a number of substances. The run-up area would be a good candidate here, because a lot of aircraft sit over the same spot for a few minutes. Although not leaking much oil individually, the collective effect of a drop or two from dozens of aircraft can produce a noticeable stain. Then along comes a little rain to lift this out of the pavement surface, and it gets slick in a real hurry. Touchdown zones are even worse, because they get oil shaken loose by the landing impact and a lot of rubber dust, too. At some really busy airports, they have to periodically go out and grind this stuff back down to the pavement. Once again, just add water for a nice, slippery mess! In most cases, a good hard rain will wash away most of this, so most occurrences are as the rain starts or after a light shower or heavy dew." You've probably notice this effect when you turn onto a damp runway in the touch down zone and get that nose wheel skid sensation / noise. |
The old rule may not be adequate. ‘Hydroplaning of aircraft tires‘. https://core.ac.uk/download/pdf/53034583.pdf See Figs 1 & 2. “It is concluded from this analysis that modern tires have lower hydroplaning speeds than previously assumed.” Also re the accident, whereas a wet runway / hydroplaning might have led to the poor deceleration during the latter part of the landing, it is also possible that a change in runway surface would give similar indications. Note the the report’s discussion of minimum runway surface friction and ‘slippery when wet’ (page 15) and the aerial photo of the runway (fig 9, page 16). Also; https://www.skybrary.aero/index.php/Aquaplaning |
Didn’t Hobart have a Virgin 737 put its nose in the grass after treating the runway with the same chemical as Christchurch did? Maybe six kor seven years ago? |
Thanks Goldenrivett and PEI3721 - Appreciated.:)
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Capn, #26 Thanks. The QRH is of greater interest. Presumably the table ‘ref distance’ is used once all of the relevant corrections have been applied; then calculating and adding the safety margin >15% to give the expected landing distance. The chart illustrated on page 25 looks like the basic Boeing format for ‘actual’ distances, which is open to debate whether this is realistically achievable, even before considering misjudged runway braking conditions. As I recall this was part of the background to the FAA’s recommendation for >15% additive (SAFO) because the assumptions in calculating the baseline distance were ‘best piloting performance’ in every circumstance. The remaining question is what was the basis of the QRH distance used in the accident; has it changed (to OLD), and if so what additional distance is given by the assumptions in the OLD calculation; and are crews aware of any change? |
From the ATSB website; ATSB Executive Director, Transport Safety, Mr Nat Nagy said the incident demonstrates how the subjective nature of runway surface reports and braking actions reports can contribute to the risk of a runway excursion. “There is little standardisation between how pilots, industry and regulators describe runway surface conditions,” Mr Nagy said. “Information provided to pilots is often from multiple sources, with undefined terminology that may only be relevant to a certain time or type of aircraft. “To overcome this, the International Civil Aviation Organization will be adopting the United States Federal Aviation Administration’s (FAA) runway condition assessment matrix in November 2020.” The Civil Aviation Safety Authority (CASA) have advised the ATSB they anticipate aligning their runway condition definitions with ICAO post 2020. “Until then, pilots are advised to apply a conservative approach when relying other pilot reports for runway surface conditions, in particular, when the conditions are considered damp,” Mr Nagy said. Additionally the ATSB found, along with the FAA, that the 15 per cent in-flight safety margin applied to actual landing distances may be inadequate under certain runway conditions. CASA has advised they intend to evaluate this finding and update the corresponding advisory material accordingly. |
Originally Posted by Safetypee
The chart illustrated on page 25 looks like the basic Boeing format for ‘actual’ distances
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parabellum, et al. More recent and in-depth (!) information, tyres, anti skid, runway texture, here:- (19MB) https://www.futuresky-safety.eu/wp-c..._D3.3_v2.0.pdf With this greater understanding and issues of different tyre construction, variable runway surfaces, etc, I wonder if the current (updated) wet landing distance calculations still provide the same landing distance safety margins previously assumed |
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