I cannot find the reason not to override the autobrakes but my guess is that the auto system is going to be evenly applied on both sides and more controlled. But then I have read some that would say manual is better than auto and so I will not go down that road sir..
Time for me to read that Airbus Journal and do some swotting...
If footbrake pressure exceeds a pre-determined value the autobrakes are disconnected and manual braking is restored with full anti-skid protection.
I've just watched the ppt again and think the emphasis is on reduced fuel and DMC (Direct Maintenance Cost).
It suggests that overiding the autobrakes increases the brake temp. and increases tyre wear and the subsequent costs. They also say that for performance issues (I guess earlier runway turn off ?) then you should consider using Flaps full etc.
Autobrakes let the HUMAN concentrate on DIRECTIONAL Control
"... why ... not ... override autobrake to shorten the landing distance?..."
Slick Rwy and tricky winds, threat is Rwy Excursion (off-end, off-side): Manufacturers want the Autobrake System available to REDUCE the human's workload (so don't trip-off the ABS with toe-pressure).
SLICK RWY, tricky WINDS -- Stopping versus STEERING: RUDDER PEDALS -- Heel & Toe COMPLEXITY during roll-out :
On slick rwy, use of the Autobrakes helps the PILOT to simplify his PEDAL inputs to keep DIRECTIONAL CONTROL (while the autobrakes work the stopping problem). Cornering versus Stopping is a frictional give&take: With MANUAL Braking, the pilot works BOTH the Rudder PEDALs and the TOE-BRAKES and that can add complexity to the roll-out. Yawing moments from differential Toe BRAKES are very high on dry rwy; during high-speed roll-out on SLICK RWY, the directional control [yawing moment] is better from RUDDER surface (since there is a lack of NWS cornering Yawing Moment).
From AAR/01-02 [AA / LIT]
“2.6.3 Spoiler and Braking Systems Procedures
At the time of the accident, American had not adopted Boeing’s MD-80 ... autobrake procedures. Boeing’s procedures recommended ... the use of maximum autobrakes for landings on wet or slippery runways.... if the spoilers had deployed and the flight crew had selected maximum autobrakes for the landing, initial brake application could have occurred about 4 seconds sooner....”
Finding #15, from Conclusions
“15. Automatic brake systems reduce pilot workload during landings in wet, slippery, or high crosswind conditions.”
Just in reference to Autobraking and an above comment...If you are in Autobrake and you press the brake pedals and request an input that is greater than the autobrake commanded rate of decel, you will indeed disengage the Autobrake and will be manually braking...BUT you will NOT lose Anti-Skid. You Cannot have Autobrake without Anti-skid.
Location: Correr es mi destino por no llevar papel
In the A320 series the MAX autobrake is not recommended for landing if you read the QRH. And it cannot be set in the air anyway.
Must be new modification. We used to operate 320 with serial no in low 200s which once landed with max autobrake armed. The crew swore it was genuine mistake and they absolutely were not bent on experimenting. They were surprised by rapid derotation immediately following touchdown, but otherwise rollout was normal and nose gear suffered no damage.
Re “Does the aircraft need to be able to stop on the MED setting if it is to be allowed into a particular runway?” (#8) All aircraft must be able to stop within a % the total runway length available depending on the conditons (60% for a dry runway). There is no mechanism for the autobrake, at any setting, to guarantee any particular stopping distance. Auto brake can only demand a particular, selectable deceleration by modulating the brake pressure and then it is limited by the anti skid system. The stopping distance depends on the runway conditions, the runway friction, and whether any additional deceleration device is available, e.g reverse thrust, spoilers, or airbrake. Where the runway friction is very low, then even with max brake pressure, the selected deceleration may not be achieved. In these conditions the aircraft may not be able to stop safely, i.e. contaminated/slippery runway operations not allowed at LCY.
Have seen time and time again, one gear's brakes hotter than the opposite. Uneven pressure has to degrade systems and performance in my assumption. Human factor of being one leg/foot dominant is probably normal.
Wonder if this is a significant factor... Or possibly they want to create a controlled environment to further remove human interaction from the operation of an aircraft as is the goal of the industry, to prove automation is safer. Airbus is big on this stuff.
Paradoxically, there can be a situation where use of autobrakes can get you into trouble and that is when a directional control problem occurs during a landing roll-out on a slippery runway with a crosswind. The Boeing 737 FCTM gives advice on how to correct a sideways slide when using reverse thrust. It advises the pilot to correct back to the centreline by reducing reverse thrust to reverse idle and release the brakes. In practice this is quite tricky since from full reverse to idle reverse of 23 percent N1 can take as long as 15 seconds to reach a stabilised idle. At the same time a quick stab on the rudder pedals to disengage the autobrakes can cause a further yaw unless the autobrake switch is used to disengage the system.
It could be argued if a strong crosswind is present on a slippery runway it might be worth considering not using autobrake at all but rely on tactile feel of manual rudder pedal pressure to know when to ease off braking in response to unwanted yaw. Boeing go on to say that when re-established on the centre-line apply max braking and symmetrical reverse to stop the aircraft. But you have to keep in mind that it takes several seconds to spool up from idle reverse to full reverse.
Quote: In the A320 series the MAX autobrake is not recommended for landing if you read the QRH. And it cannot be set in the air anyway.
Must be new modification. We used to operate 320 with serial no in low 200s which once landed with max autobrake armed. The crew swore it was genuine mistake and they absolutely were not bent on experimenting
Newish, we've operated A320 series from MSN0006 (No 6 off the production line) to 2009 deliveries and you could land with Autobrake MAX on the earlier aircraft but not on the later one. By no means definitive but the first aircraft I recall seeing this change was around 2004/2005 but please don't quote me on these dates.
As an aside, the early aircraft didn't have brake fans whereas the later ones did and there was a noticeable decrease in brake wear on the later ones. It was noticed that with the carbon brakes and number of applications affecting wear more than braking force applied, pilots weren't worried about heating them too much so tended to use them less timidly hence less appications, especially on short (30-35 minute) summer sectors with only 40 minute turnarounds.
Do you consistently land with a crosswind from one particular side?
I am left footed and most often find that it is the "downwind" brake assembly is hotter on arrival at the gate in what I can only assume is the extra braking required on that side to prevent weathercocking in the landing roll.
Nothing to do with left or right "footedness" in my experience.
Of course, other factors like taxi technique and which direction the wind comes from in your taxi to the gate will also affect brake temp differential etc.
As for max mu, my fcom says antiskid will allow wheelspeed down to .87 of aircraft reference speed (furnished from adiru's). In case of failure of all 3 adiru's, reference speed is equal to the greater of either main gear wheel speeds.
All I know is in 7 years landing in Tegucigalpa Honduras, noted as one of the most dangerous airports in the world if you google it, you can land shorter using manual braking with full anti skid using the highest normal short of max setting. auto brakes 4 was what the procedure said was required for landing. The 757 stopped very well and with that short runway with a cliff at the end and a hill on final you didn't want to not do it right. That was the only airport we were required to land with AB 4 set. Our manual told us that. I have never flown an Airbus but assume it is the same. I don't think they can certify an airliner that you can push on the brake pedals enough to disable the antiskid.
So, even with anti skid working, if the the runway surface has reduced grip due to either contamination or poor maintenance the system could make you think the wheels have locked, as retardation will not be as expected?