A320 landing distance without failure

[simyoke]

Hello, folks!
While going through QRH the other day, I realized that the second note seems a bit strange for "LANDING DISTANCE WITHOUT FAILURE" tables.


Each LANDING DISTANCE WITHOUT FAILURE table has two notes, one of which is related to WEIGHT CORRECTION;


For Dry runway,
(2) Weight correction: if CONF FULL, subtract 10ft per 1klb below 150klb.

For Good runway,
(2) Weight correction: if CONF FULL, subtract 20ft per 1klb below 150klb.

For Good to Medum runway
(2) Weight correction: if CONF FULL, subtract 20ft per 1klb below 150klb.


Let's say that we are using autobrake low with flaps full, then the reference distance is same for all three runway conditions mentioned above.


If you follow the aforementioned note, then Good to medium and
Good runways produce a shorter landing distance than Dry runway.


Dry runway is supposed to give us shorter landing distance than Good or Medium, or so my common sense tells me.

[Fursty Ferret]

Reverse thrust isn't counted on a dry runway in the IFLD tables, hence the difference.

[compressor stall]

"Google getting to grips with aircraft performance" a book by Airbus. Chapter 3.2.1 will help you.

[simyoke]

Let's do a rather simple arithmetic here.
Assuming all the parameters same for both dry and good to medium runways;


conf: full
auto brake: low
weight: 100000 lbs
spd: assume no speed increment is needed
alt: 0 ft
wind: no wind
temp: ISA
slope: 0
reverser: both are operative


Let's add or subtract as directed in the table ....
,which yields


5870 ft for dry runways
5350 ft for good to medium runways

To me, it seems counter intuitive.
Looking up GETTING GRIPS WITH A/C PERFORMANCE, One thing that I can think of is precipitation drag.


The major difference comes from the second note since GOOD TO MEDIUM runways let you subtract twice as much as DRY runways
for weights under 150000 lbs.

[FlightDetent]

Reverser is not taken into account (regulatory requirement) on dry runways.

The delay after touchdown before the autobrake LO comes into operation and the effect of the reverser in the meantime should explain the difference.

Also. you need to apply, I think, DRY/WET check, which tells you to disregard any wet runway results if they come to a better figure than those calculated from dry.

As far as I remember airframe drag (from spray) only enters the equation on contaminated runways.

[simyoke]

[QUOTE=FlightDetent;9535569]Reverser is not taken into account (regulatory requirement) on dry runways.

But when you do the landing distance calculation the table has reverser column.

[QUOTE=FlightDetent;9535569]Also. you need to apply, I think, DRY/WET check, which tells you to disregard any wet runway results if they come to a better figure than those calculated from dry.

DRY/WET CHECK is done for take off performance as the FCOM says in the take off performance section due to 15 foot vs. 30 foot screen height for dry runways.
QRH does not say doing dry/wet check for landing distance calculation, though.

[FlightDetent]

Both your points sound valid.

[Citation2]

There is no dry check for landing. Dry check is for take off .

[simyoke]

Both your points sound valid.

Thanks for your input.

It takes more than 500 feet to stop the airplane on a dry runway than on a runway with less friction???...it just doesn't make much sense to me at all.

[Goldenrivett]

it just doesn't make much sense to me at all.

If you look at Landing Distances with Maximum Manual braking, then the dry distance is the shortest. That makes perfect sense to me.

[Feather44]

I've just checked with A320 FlySmart EFB (Ipad).

The results with autobrake for dry, good, good to medium are similar. but from medium the Ldg distances start to increase.
The results without autobrake, the Ldg distance increases conventionally. (dry Ldg distance is shorter that good, good is shorter than good to medium etc...)

My guess: the autobrake system (the decel rate feature) is fantastic, it can cope pretty well with slightly degraded rwy conditions.

[simyoke]

If you look at Landing Distances with Maximum Manual braking, then the dry distance is the shortest. That makes perfect sense to me.

Yes that is when max manual braking is used, however, max manual braking is rarely used and when you put in the parameters that are given as above for landing distance calculation, it would look at least a bit strange mainly beacause of weight correction factor being twice as much for runways with less friction.

[Goldenrivett]

weight correction factor being twice as much for runways with less friction.

It's not friction causing the difference - it is reverse credit.

FlightDetent explained in post #5.
You are not simply comparing only the effect of brakes and runway friction.

Autobrake Low simply sets a deceleration rate which might be achieved initially with reverse thrust and the brakes hardly doing any work.
If the published Landing Distances assume no reverse credit on a dry runway but use of reverse on the others, then your observations make sense.

See https://www.fly19.net/wp-content/uploads/2013/05/In_flight_landing_performance.pdf

[simyoke]

It's not friction causing the difference - it is reverse credit.

FlightDetent explained in post #5.
You are not simply comparing only the effect of brakes and runway friction.

Autobrake Low simply sets a deceleration rate which might be achieved initially with reverse thrust and the brakes hardly doing any work.
If the published Landing Distances assume no reverse credit on a dry runway but use of reverse on the others, then your observations make sense.

See https://www.fly19.net/wp-content/uploads/2013/05/In_flight_landing_performance.pdf

A320 Landing distance calculation tables for all the runway conditions have reverser column, that is, you can decide whether you use it or not. As I mentioned earlier, the 500 foot difference is still there with all the parameters set at the same values for different runway conditions.
You might want to look up the landing distance tables contained in QRH which can be found in the link you provided.

[Goldenrivett]

You might want to look up the landing distance tables contained in QRH which can be found in the link you provided.
I did and get the following figures which all make perfect sense.

Weight 66Tons Autobrake Low, No Rev
Dry Ref Dist = 1950m Correction -10m/Ton below 66Tons.
Good Medium Ref Dist = 1960m Correction -10m/Ton below 66Tons.
Medium Ref Dist = 2140m Correction -20m/Ton below 66Tons.
Poor Ref Dist = 3540m Correction -30m/Ton below 66Tons.

The bigger corrections / Ton below 66 Tons for Medium - Poor conditions will be due to Airframe drag etc. which has a longer time to produce an effect due to the longer Landing Distance.

I suspect your figures have a printing error.