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744 RTO

Old 9th Nov 2021, 03:13
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SRS
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Question 744 RTO

Our training program requires us to apply full brakes or monitor RTO during the stopping progress.
There is a note in our manual ( not Boeing ) saying manual braking is superior to RTO.
To me that does make a lot of sense because the RTO feature applies 3000 PSI to both brakes, and no matter how
hard you apply manual brakes that is the system limit.

Am I missing something ? Is the note mistaken ?

Thanks all in advance for your time.
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Old 9th Nov 2021, 05:08
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The 777 initially had a note, that I believe was misunderstood, stating that Max Manual Braking exceeds Max Auto. This statement gives support to your interpretation, but may not be what was intended?

I also recall doing an RTO exercise that 'demonstrated' that Max Manual stopped quicker than RTO. I'm just not sure it was very scientific…
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Old 9th Nov 2021, 07:30
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CCA
 
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As above, max manual can achieve more than MAX AUTO.

MAX AUTO - 3000psi Decel 11 ft/sec
4 - 2050psi / 7.5
3 - 1750psi / 6.0
2 - 1500psi / 5.0
1 - 1300psi / 4.0

RTO is max available pressure with no decel target which could be 3000+ and would theoretically equal max manual.

Having tested the RTO function in the aircraft I can assure you, to get RTO to disconnect the pedals are held at the stops for what seems like several seconds before the AUTOBRAKE CAUTION appears, thereís no sense of increased braking over and above RTO.
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Old 9th Nov 2021, 09:31
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In the real world RTO braking will always exceed max manual coz it's applied at once, within ramping limits, and not modulated in any fashion - think crosswind with difficulty applying full pedal travel just for starters.
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Old 9th Nov 2021, 10:52
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Can you post the note in your manual? Is the manual company written or Boeing? Every Boeing with autobrakes I have flown RTO gives the maximum possible braking. Max auto brakes is different from RTO and you can out brake it using manual brakes but it won’t be easy.
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Old 9th Nov 2021, 13:22
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Auto-brake, for info:

http://www2.alpa.org/DesktopModules/...2156&Tabid=256

Slide 11 - re autobrake tuning, modulation - efficiency, compromise.

Slide 21 manual brakes, “The antiskid system stops the airplane for all runway conditions in a shorter distance than is possible with either antiskid off or brake modulation”, i.e. max manual better than ‘max’ auto which is modulated.

Note 1 the presentation relates to flight testing, not commercial flight operations or procedures.

Note 2 the performance data terminology and calculations relate to the AFM - flight testing. Recent operational changes should require use of ‘Operational Landing Data’ replacing the previous Boeing ‘actual’ data (QRH); this should also be compatible with the new Global Reporting Format for runway condition - effective this month (Nov 2021).
https://www.caa.co.uk/Commercial-ind...orting-system/

Note 3 beware simulation, it may not accurately represent the aircraft in all conditions - runway surface, contaminant, etc; … unless the simulator is hooked up to the equipment in slides 10 & 12

General info: http://www.icao.int/SAM/Documents/20...0Aeronaves.pdf

CCA re use of max manual vs auto RTO; is the alert a real time indication relating to brake pressure or just indicates a system state - no computation / modulation. The information above suggests that because max manual provides better (instantaneous) performance, it may be better to use manual in extreme circumstances; i.e. abnormal runway surface / conditions, alleviating interaction of reverse with ABS on very slippery runways.
However, operators may wish to balance these extremes with the realistic, but rare need for RTO, and choose Auto Brake as standard lest pilots are unable to perform as expected by theory.
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Old 9th Nov 2021, 15:31
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IF you hold the brake pedals all the way down, it is the same as RTO. RTO IS max braking, which is more than MAX Auto.

From the 744 FCOM:

Rejected Takeoff
Selecting RTO (rejected takeoff) prior to takeoff arms the autobrake system. The RTO mode can be selected only on the ground. The RTO autobrake setting
commands maximum braking pressure if:
• the airplane is on the ground
• groundspeed is above 85 knots, and
• all thrust levers are closed
Maximum braking is obtained in this mode. If an RTO is initiated below 85 knots, the RTO autobrake function does not operate.
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Old 9th Nov 2021, 23:18
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RTO will out perform max manual braking.(Humans just arenít that good, flight test adds a one second delay to account for this) When it comes on itís instantaneous the nose drops noticeably as soon as the thrust levers hit the idle stops, reversers will have no affect on RTO breaking other than reduce the brake temp.

Edit: below as mentioned RTO with reverse helps.

Last edited by CCA; 11th Nov 2021 at 07:32.
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Old 10th Nov 2021, 09:31
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Seem to remember an analysis of RTOs where Pilots thought they had applied max breaking but, in fact, they hadn't pushed the pedal all the way to MAX and so were somewhat short of max pressure. In training many guys/girls had their seat too far back to mechanically (body) apply MAX full pedal deflection.
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Old 10th Nov 2021, 15:11
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I wonder if the OP is confusing the RTO function with max auto. As I recall max auto is less than max manual. However, in the RTO situation the SOP would be to utilise the RTO function which as others have said will apply brakes quicker than the pilot can. Not least as he/she may be preoccupied containing the swing! In that respect manual braking is only used as a backup to the RTO function being inop.

Of course the low speed RTO where the engines will still be trying to set TOGA and the RTO function has yet to be activated is an eye opener. “Too quick for the tiller, too slow for the rudders” as an experienced trainer used to say. Oh and don’t forget to click out the autothrust.
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Old 10th Nov 2021, 20:37
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CCA

Reversers will not have any effect on stopping distance on landing with Autobrakes, since they are set to a decel rate. However, on an RTO the reversers WILL have an effect, since brake pressure is not modulated and reverse thrust will supply additional deceleration.
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Old 11th Nov 2021, 07:30
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Absolutely correct, sorry was thinking certification with and without reverses.
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Old 12th Nov 2021, 02:13
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My experience of demonstrating Max Manual braking versus RTO in the simulator is that RTO normally wins because pilots invariably ease pressure on one brake as they struggle to keep straight.
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Old 13th Nov 2021, 22:18
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safetypee

I think you are getting confused with anti skid performance verses no anti skid. Anti skid will always outperform no anti skid or pilots attempts to simulate anti skid via pedal modulation. The anti skid issue has nothing to do with the discussion as all braking methods being discussed utilize anti skid including RTO and max auto.
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Old 13th Nov 2021, 23:04
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Sailivi767,
The (Boeing) reference, slides and quotes were for general info.

No confusion with anti skid.

The main difference in the thread debate is with the mode of operation of an auto-brake system; those functions with auto-brake modulation for landing, i.e. max auto (which is less than max manual); and an RTO function without modulation where the applied brake pressure (maximum) does not differ between auto and manual.

The differences in RTO technique, auto/manual have been debated - refer to manufactures advice and check the small print re performance, and any change with technique.

There may be confusion if some aircraft refer to anti-skid as being ‘modulated’ - (it is) - choice of words; also if modern auto-brake and anti-skid systems are fully integrated.

The after-comments re ABS, reverse, and slippery runways relate to modulated systems during landing - general info.

The wording in the OP question (not manufacturers) may have mistaken the difference between Max and RTO, as above.
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Old 14th Nov 2021, 12:54
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Auto brake and anti skid systems are integrated including RTO on a Boeing. If not RTO would lock all the brakes up and especially on a wet surface increase stopping distances substantially. RTO is also quite violent especially at slower speeds and lighter weights. It’s nothing like max auto. At higher speeds and weights it will almost certainly require replacement of all wheels and brakes and it’s likely a brake fire may result.
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Old 14th Nov 2021, 19:24
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This may be less to do with the ability of the system to brake and more to do with timing.

The autobrake won't kick in until the thrust levers are at idle, which means that for an RTO where the pilot applies maximum brakes simultaneously with closing the thrust levers they gain a fractional head start on the autobrake. I can imagine this being a slower process on the 747 given its age and architecture.

The above is just a guess, admittedly.
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