I shall try to answer/comment on a few things mentioned in previous posts,
Backtrack said:-
As I understand the system, setting the park brake closes a valve in the return lines, thereby trapping fluid (pumped at 3,000psi) between the brake units & the hyd reservoirs. This makes obvious sense for when the a/c becomes depowered as there will be no pressure in the normal lines & without this trapped fluid, the a/c could move.
For all intents and purposes, pretty well spot on the money with the above.
Backtrack also said:-
Because Boeing's QRH drill recommendation is to only 'consider setting the park brake if an evacuation becomes necessary.' ( I've paraphrased this last bit from the QRH).
What I don't understand is why Boeing don't feel this is a requirement at the end of an RTO. The first item on the new, by reference, evacuation checklist is to set the park brake. Why only now? What's the technical objection in setting the park brake at the end of an RTO & releasing it if it is decided to taxi clear?
Spanner Turner
Do you have an expert's view on this?
Okay, I’m now stepping out of my territory (i.e I’m not a pilot, don’t have access to your QRH or SOP’s and am only commenting as I’ve been asked to and based on my previous experience in dealing with the aftermath of RTO’s)
What I don't understand is why Boeing don't feel this is a requirement at the end of an RTO.
Could I venture a guess (a second guess at that) as to why Mister Boeing has come up with this?
I think the point here is that there is a BIG difference between an RTO and an Evacuation. Not every RTO ends in an evacuation. A “Rejected Take-off” is just that, a decision made after commencing the take-off roll that the take-off should not be continued. The reasons for this decision are many I’m sure you’ll agree. The decision alone to abort a take-off in no way implies that the passengers/crew are in danger and need to be evacuated, it doesn’t even suggest the aircraft was incapable of flight, it purely means that the pilot has decided that it be prudent to abort and slow down/stop the aircraft.
Some of the reasons for aborted take-offs I’ve seen in the Tech Log are;
Cargo door light illuminated during roll.
Capt’s #2 window handle moved from ‘Lock” to ‘Open’ during roll.
Nearby aircraft on taxiway reported ldg gear pin streamer visible on the Tower frequency
Extreme nosewheel vibration.
Runway incursion by ‘Safety’ vehicle.
Runway incursion by other aircraft.
Captains Mother calls the Tower to say that ‘Little Johnny’ has forgotten his lunch.
My fleet also has a bad history of Engine ‘Bleed’ defects on the 767 causing RTO’s.
In all the above instances I’ve never heard of my flight crew actually letting the aircraft come to a complete stop under autobraking and wanting to “Park” the brakes on the active strip. In reality, the defect is noticed, decision made to reject, thrust levers retarded, autobrakes apply, the aircraft speed is slowed down via autobrakes, whereupon manual braking is applied (like a normal landing) which disarms the autobrakes, and the aircraft vacates the runway.
The first item on the new, by reference, evacuation checklist is to set the park brake. Why only now?
As above, the difference here is that you are “evacuating” the aircraft. This MAY be after an RTO, but it may just be after you’ve taxied out of the ramp. If you’ve decided that all the SLF need to try their luck down the slippery slides then it is best that you set the Park Brake. This stops the aircraft from moving as the victims hurtle down the chutes towards the unforgiving tarmac. (It also stops some nasty abrasion on the underside of the slide)
What's the technical objection in setting the park brake at the end of an RTO & releasing it if it is decided to taxi clear?
I’m sure there’s no technical objection. If a cargo door light comes on on your next take-off roll (or dear old Mum calls to say you’ve forgotten lunch) and you decide to abort, I’d say you’d be quite within your rights as Captain to come to a complete halt on the strip and to set your Parking Brake before deciding the next course of action.
None said,
Back to the technical questions, here is some more info:
The AMM does not say if the system brings the aircraft to a complete stop, however it also does not say that the system releases the brakes nearing full stop.
If you have a look at my post #11, on more than one occasion are the following words
"Autobrake application in the RTO mode applies full system pressure."
Just like your car, if you supply “Full system” pressure to the brakes, by default the aircraft will come to a complete stop. (Ideally before hitting an immovable object)
None said,
If the airplane speed drops below 7.5 knots, the system provides no brake release for the antiskid signal.
This is 100% true. If there was no low speed drop out then the aircraft would never come to a complete halt as the anti-skid would see the wheels as locked and release pressure from the brakes.
In reality this means that when IRS speed falls below 7.5kts, the wheels may lock-up depending upon pressure supplied.
None said,
In addition to the F-STOP items, the RTO is disarmed when one of the air/ground signals shows the air mode (Power is removed from the latch, and the switch turns to the OFF position). This is what you see after takeoff.
Well no actually, see post #11, RTO is disarmed for the reasons listed
The autobrake system remains latched in the RTO position until oneof the following occurs:
• Selector switch is placed to OFF.
• At least one air/ground signal indicates air mode.
• There is a loss of autobrake power.
The fstop items are ones that ‘release’ the braking pressure – not disarm the RTO.
Spanner Turner ... thank you for such a clear reply. Be a little forgiving though! ... we mere pilots do not have access to the engineering manuals.
That may be true my friend, but us mere engineers don’t have access to bright shiny aluminium tubes that can soar through the skies and in a few small hours, transport oneself to far-off exotic locales and new worlds of undiscovered bars, pubs and nightclubs and experiences.
During my 'workday' I venture no more than 5 Nm from my home.
Boring.