I flicked the annunciator test switch to show you all the pretty lights associated with the autobrake system.
Them’s the Brakes…
This post is for you! I commented in my previous post I received your letter and your book was sent Friday. Enjoy the read!
Yes, aircraft implement anti-skid and the idea spilled over to cars which you well know as ABS (anti-lock brake system). This topic got me into the books. Well I had to do something on the six hour flight to San Francisco. J
One thing I didn’t realize it did not activate below 20 knots so if I brake and skid a little on some ice, I may leave some rubber behind.
The anti-skid system compares the speed of each main gear wheel (tachometer) with the speed of the aircraft (reference speed). When the speed drops below .87 times the reference speed, the system orders brake release. The Airbus is like an onion, it has many layers of complexity. Maybe I should call it the Airbus Onion. (I also know a few people who have multiple layers)
The Airbus autobrake system has four simple settings: off, low, medium and maximum. It does not arm below 72 knots so in a reject we must be cognizant of this limitation and apply manual braking. We take off with the system in “max.” So with max reverse and max braking... “weez a stopping in a hurry.” Lol
“Lo” setting activates in 4 seconds after the main’s touch and the ground spoilers deploy. This “lag” is a little too slow if we are landing at airports with a “get on, get off” operation. Myself, I land using “low” and with idle reverse, but if need be, I will apply manual braking. I also use the system in crosswind landings. Again, it takes patience to wait for the autobrake to kick in and to determine if you will make the the appropriate turn off.
We tend to maximze brake use over reverse because our brakes are leased and it saves wear and tear on the engines. The glitch is... the brakes warm up in a hurry and carbon brakes work better with one or two smooth applications instead of multiple inputs.
"Medium" kicks in 2 seconds with a deceleration rate of 9.8 ft/sec squared versus 5.6 for low. We tend to use medium in places like USS La Guardia (USS because water stares at us on three of the four runways). J Also if the runways are contaminated (i.e slippery) or landing in low visibility we employ medium.
"Maximum" is NOT recommended for landing because anything not strapped down would end up in the front of the airplane. And that’s where we are!
The Decel (deceleration) lights activate when at 80 % of the selected rate.
The nose wheel steering disengages above 70 knots. The nosewheel rudder pedals disengage above 130 knots…after that you are steering with only rudder.
So what happens when we reject around 70 knots due to an engine failure? The autobrake is not armed, max reverse is less effective at low speeds and the nosewheel rudder pedals only allows a deflection of +/- 6 degrees. Translation, if you don’t get onto the nosewheel steering (hand wheel) and brakes…it’s off into the tulips you go. Yes, sometimes a low speed reject is much slippery than a high speed.
And how many rejects have I had after accumulating thousands of take offs?
Zero! Okay... I had a couple of very low speed rejects due to NON mechanical issues but that’s it!
The end of February will see Captains Doug in the simulator and rest assured I will be yelling “reject” as part of the ride.
It's Valentines Day and I hope all of your significant others don't say to you..."Them's the Breaks" in the romance department. :)