Required landing distance

[Seminole27p]

Hi, in regards to figuring out the factored landing distance, can someone tell me where does the 1.67% comes form. I am clear as to "the a/c must be able to land in 60% of the effective runway (15% more if the rwy is wet) However, I am trying to figure out where the 1.67% number comes from.

Thanks!

[john_tullamarine]

Smoke and arithmetic mirrors ..

The 1.67 is a certification standards thing for heavy aircraft.

Either you can look at the

(a) actual landing distance of which the aircraft is capable and factor that by 1.67 to arrive at the minimum distance required or

(b) distance available and factor that by 0.6 to give a minimum capable landing distance for the conditions.

Lo and behold .. 1.67 = 1/0.6

If your AFM includes the factor, you just read the chart. If it doesn't, you either go via (a) or (b). In operations, (b) is probably the more usual as you will know the availabe LD and wish to figure the limiting landing weight.

Not really 1.67% .. rather either a 1.67 factor or 167%.

[HazelNuts39]

The 60% and 15% are in the operating regulation, not in the certification requirements.

The origin of the 60% is hidden somewhere in a distant past. The 15% on wet runways for turbojets is more recent, late 60's or early 70's IIRC (Amdt. 121.9, Eff, 1/15/66).

[john_tullamarine]

Depends on which Authority and the period in which you are looking.

For the US, you are, of course, correct.

Australia, for example in years gone by, put the requirement in the certification sections (ANO 101.5 - BCAR certification - and 101.6 - CAR/FAR certification) as was the philosophy in a number of differences with the major Authorities.

Memory's scratching a tad and I would need to dig out a couple of original AFMs to double check on landing distances .. but I recall that the same applied in the UK under the BCARs.

I resile in mortification :O ... but it's minor point in the overall pursuit of pedantry, I would suggest.

As to origin, my guess is ICAO around the 50s. One of our number here in the sandpit has a well-thumbed copy of the then report of wise words and, on occasion, quotes selectively from it for our benefit .. we can but wait and see on this occasion.

[Seminole27p]

Thank you guys for taking the time to explain!!!

[LeadSled]

Seminole27p,
An easier way to look at it is that the FAA required that the demonstrated length be 60% of the published length, ie: the buffer or fudge factor was 40%.
The first time I worked with these numbers, they were in SFAR 422B, which preceded FAR 25.
For may years, the 15% (on the 100%, not the demonstrated length) was a recommendation for a wet runway, ie: Captain's discretion. I won't even try and go into the details of "wet runway" definitions, but CASA want to make 15% mandatory if it is raining --- ie: no recognition of the fact that all our major airport runways are grooved, or have a porous overlay --- ie: the equivalent of a dry runway, as long as there is no pooling of flooding.
In a wonderful demonstration of in-depth knowledge of these numbers, and involving a small Cessna jet, a "well knows civil aviation safety authority" demanded the Cessna published landing lengths (already factored) be factored again (in the "accepted operations manual") by a second 1.67, and the result be further factored by 15% for a wet runway.
This resulted in the farcical situation that the only runway that this little aeroplane could use, in the wet, in the state of NSW, Australia was RW 16R/34L at YSSY, and there were precious few it could use in the dry.
It took quite some time to get the message across that this approach was just plain wrong.

[CliveL]

JT

Memory's scratching a tad and I would need to dig out a couple of original AFMs to double check on landing distances .. but I recall that the same applied in the UK under the BCARs.

When I started in the late 50s the UK authority (ARB) used a 'rational landing distance' based on a reference wet runway. Certification braking tests were preceded and followed by runs of a "Mu-meter" to measure actual runway friction so that measured distances could be corrected to the reference surface. If I remember correctly the regulations also included allowances for higher approach speeds and touchdown point.

FAA flirted with a similar concept in the 60s or 70s (memory fades!) using a 'diagonally braked vehicle' to measure the friction, but IIRC they came to the conclusion that it would be too difficult to classify every runway in use, particularly since the friction varied with time (rubber pollution amongst other things) so they abandoned the idea.

[LeadSled]

If I remember correctly the regulations also included allowances for higher approach speeds and touchdown point.

CliveL,
That's how I remember it as well, in general terms, perhaps somebody with a better memory or library could expand. Doing ARB Performance A was a long time ago.
What I also remember was that the ARB factors were smaller than CAB/FAA figures, and the bottom line was that the final figures wound up being very similar ---- a quite complex (but line operationally realistic) test requirement with small buffers, versus a simple test requirement with a big buffer.
Tootle pip!!

[CliveL]

Leadsled
What I also remember was that the ARB factors were smaller than CAB/FAA figures, and the bottom line was that the final figures wound up being very similar

For our aeroplane(s) the ARB distances always came out a bit higher than the factored FAA distance, which was a marketing disadvantage so we ended up doing both if we wanted certification in other than the UK.

But the FAA couldn't have been altogether happy with their distances or they wouldn't have gone to all the expense of their DBV programme I think?

Which, since they never changed anything, prompts a question as to why? Possibly better brakes and more reliable reverse thrust have given better statistics (and yes I know you can only get credit for that if you can show it to be reliable - but that is for certification distances not the reality of everyday operations).

OTOH aircraft still go off the runways from time to time so it can't be all sweetness and light ...........

Cheers

[baobab72]

Hi
In plain english
The LFL is the distance needed to bring the airplane to a complete stop, crossing the threshold at Vref at a height of 50ft - ALD - multiplied by 1,67. This factorized LFL must not exceed 60% of the LDA or landing distance available.

Hope il helps

Baobab72

[MU3001A]

I am trying to figure out where the 1.67% number comes from.

Simple math.

Multiply un-factored landing distance by 1.67 to get factored landing distance.

3000 x 1.67 = 5000

3000 is 60% of 5000

[tommoutrie]

I think everyone accepts that the intention of mandating turbofan aircraft to land within 60% of the available runway is to increase the safety margin but its a bit strange that a linear factor is used to factor a quantity that varies with the square of the speed. The way the regulations are currently applied, the actual margin of safety is greatest when you are light and at a minimum when you are heavy. If the amount of brake energy you can exert is reduced by the most common constraint, friction (a reduction in mu), that's a linear reduction. Which means the amount of braking you get decreases in a linear fashion whereas the amount of energy you need to dissipate increases with the square of the speed.

It would be far more sensible if the same factor of safety were applied. More complicated maths but in the days of APG and flugP, who cares. It would mean you could carry slightly more fuel or land on that slightly shorter but very useful runway.

Here's a question. Is it 70% for turboprops because the effect of beta is allowed? And if so, given the failure rate of thrust reversers, will we be able to use data which takes TRs into account at some point in the future?