B777-300ER Landing OPT Max manual vs Max Auto
 

I was asked by a colleague about some OPT landing calculations and it's got me stumped!

We are using Boeing OPT on iPad for B777-300ER and using the landing-enroute calculator.

For a manual landing the max manual brake landing distance is shorter than the max auto brake landing distance (which I expect).

For an autoland the max manual brake landing distance is longer than the max auto brake landing distance (which has me stumped!)

I tried several runways and conditions (the difference is greater with dry runway) with similar results.

Guessing mode....

Rollout mode will be active guiding the aircraft down the runway. As it makes it's inputs, perhaps Boeing discovered that the pilot can't just ram down the brake pedals whilst the autopilot controls the aircraft on the centreline. When you are doing it all yourself its easier. Maybe there is an adjustment for that?

I had researched along those lines but if that was the case then these techniques would be documented in the FCTM but I cannot find anything along those lines.

From the depths of my memory JAA needed autoland distances multiplied by 115% of certified landing distance (not sure if FAA requires this). Is this correct? Maybe the OPT figures cannot be less than the 115% so they are artificially increased?

Just looked up an old Airbus document "Getting to grips with AC performance". It seems JAA (JAR AWO - now CS AWO?) used to require autoland distances to be increased by 3 x std dev of some of the airborne portion (end of g/s to touchdown) and also by 3 x std dev of ground roll. So the above 115% may not be correct.

There is indeed a margin built in as the pilot has no external reference to keep aircraft straight during rollout.This in turn results in less brake pressure being applied as concentration is diverted to keeping plane tracking centreline.

Skyjob, it’s difficult to understand the logic of a visual constraint affecting brake use.
In the very low visibility conditions suggested, then the automatic landing system should have a high integrity rollout control or guidance element, thus the pilot should not be in visual loop as you suggest.
Does the T7 have rudder fine steering? If so, then steering would be independent of brake pressure.
Manually braked landing distances do not normally consider crosswind effects - ops on contaminated runways might.

Are there any system differences involving brake application between manual and auto landing?
Any differences in the deployment of spoilers, WoW, or reverse thrust, auto/manual land?
Presumably the landing data is based on the Boeing ‘actual distance’, but is the basis of calculation the old style ‘actual’ actual or the more recent Operational Landing Distances (with/without Factors).

With fast fading memory and from a very longtime ago, didn’t autoland distances always have to assume a wet runway, and that Cat3B ops had to use autobrake.?

For a 235 ton landing weight I found this info. For what I will call a normal landing the required runway was 5615' using max autobrake while max manual braking had a significantly shorter requirement of 4462'.

If autoland was used, the max autobrake setting increased by around 300 feet to 5918' but, the max manual braking distance increased by almost 3000 feet to 7301'.

Why so much?

Change the runway condition to wet and the difference gets a lot smaller!

I find this inconsistent.

We consider anything under 4000 RVR to be a wet runway.

FWIW not that I know of and in our old fashioned tables (in EASA land) I can find no extra increments that need applying to braking distances In the event of planning for an auto land for any of our T7s...no mention of this in our FCTM either....

Interested to hear more..