FlyerTalk Forums - View Single Post

Apr 10, 2018, 7:06 am

#3000

Waterhorse

Join Date: Aug 2008

Posts: 2,065

Quote:

Originally Posted by BertieBadger

One (ok, many!) for the pilots, inspired by a post by KeaneJohn in the NEO thread about rejected take off tests.

Presumably at their simplest these tests are what they sound like, namely can you safely halt the aircraft from some defined maximum speed (above which I guess you are taking off whether you want to or not....)

I would surmise that they need to do these tests with the aircraft at MTOW since that would affect braking distance? Do they just use ballast instead of pax and cargo? What do they do in respect of fuel load - on one hand, I can see that there could be benefit to running minimal fuel, as a risk mitigation strategy in case the test goes wrong. On the other hand, presumably a full fuel load more accurately represents the weight distribution and actual state of the aircraft in the event of a 'live' rejected TO?

From what I've read, such tests have to be conducted on the brakes alone, i.e. without assistance of any thrust reversers etc? I suspect passengers - including myself - probably overrate the importance of such additional deceleration, not least because it looks dramatic

If it can be quantified, what proportion of total available deceleration is due to the brakes?

How fast can brakes alone decelerate an aircraft, and is there significant differences between say an A318 and an A380. Naively I would expect there to be so, but perhaps the big lad simply has much more braking power to counteract the weight? (but then would it run up against limits of tyre friction?)

Lastly, in the case of real rejected TO, where the pilot can make a subsequent attempt, how do they ensure that the systems are in a state where it is safe to do so? Are there sensors, or do they make visual inspections? (I'm thinking for example, where the case of the first rejected TO has "cooked" the brakes to the point where their performance is dramatically impacted, I presume you can't simply "have another go" since if another rejection was needed, the brakes would not be able to stop the aircraft).

Lot of questions, so thanks in advance for any insights.

From my dim distant memory, RTO tests are done at Max Take Off Weight, so that would need full fuel tanks and ballast weight, think they use a mix of weight in the cargo compartment and ballast on board. Usually done on test aircraft in the early stages of the test flight routine, if not before initial flight, so there are no pax seats in the conventional sense.

The tests are done with fully worn brakes to est the worst case scenario and may or indeed often result in brakes fires. There are videos easily found on YouTube of this sort of thing.

In normal landings the use of reverse thrust is not terribly important as it only reduces the heat in the brakes. Indeed the 380 only has inboard reverse, the outers deleted in development as the aircraft was too heavy and would not meet its expected and promised performance. There are a few places where the use of reverse thrust is useful but it is not essential - some failure cases may be affected by taking account of reverse thrust but it is not normally important. The brakes are what stops the aircraf and the failure of the spoilers to deploy is more worrying than a reverser fail as it is the spoilers that ensure the weight of the aircraft is fully on the wheels allowing the brakes to operate most efficiently.

Before each landing we calculate the braking distance required and whether or not we need to use full reverse..This is belt and braces stuff as we take no account of them for the braking distance. Based on the calculation we make e will alter the required amount of braking - brake to vacate would do this automatically, the 320 still requires a bit of pilot skill in this area.

In in terms of having another go after an RTO, much would depend on why you stopped and what speed you stopped from. A high speed stop would only be done for a failure which means it is highly unlikely the aircraft would be able to fly again without engineering input. A low speed stop may allow another go provided the checks did not reveal any issues, the onboard systems would all have to indicate no fault and importantly brakes within a limited temperature range. Remember that brake energy increase as a square function of speed so relatively low speeds can produce high brake temps, also at takeoff weights you may be significantly above max landing weight so again brake energy would be higher than landing due to the additional mass of the aircraft.