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There are many steps involved with getting a commercial
airliner into the air. As such, procedures have been
implemented over the years to do this as safely as
possible. One critical step in this process is to ensure that
an aircraft is permitted by air traffic control to actually do
off from any given runway*. This is crucial since
hurtling a jet full of fuel down a runway that has another
jet full of fuel landing/crossing said runway makes a large
and expensive mess. To stop this happening, clearance
from ATC to take off is part of the take off checklist, which
contains all the necessary steps. Except it doesn't always
Recently, I'll link it when I find it again, there was an
incident where a regional jet was cleared to hold (not take
off) and started the take off roll anyway. As they crested a
rise in the runway the vertical stabilizer of a Boeing 777
came into view. Erring on the side of caution/employment,
the crew aborted the take off. Seconds after, they then
received clearance, the 777 was essentially finished
crossing by this point.
The problems here were some expectation bias, but also
because the process of getting clearance is essentially
verbal via the radio, and passing that point on the checklist
relies entirely on one/both crew remembering if they
heard that clearance. This is where the expectation bias
can creep in. If you do the same trip 100x, and you hear
clearance to hold at the end of the runway then almost
immediately clearance to take off, you may think you
heard the second part just because 100x before, you did.
The problem being, there's no visual reference like you get
with nearly every other checklist item. Other items, such
as the strobe light that should be on before take off, you
can check by looking at the switch. So why not make
clearance the same?
In this way, while making the radio call to obtain take off
clearance, your finger can be on the switch... as you hear
it, you flick the switch. Both pilots now have visual
reference as to whether this has happened. But, we can do
more than this. If this switch were linked into the avionics,
in the "no" position, several safety interlocks might be
added, for example, engine thrust might be limited to
30%** to prevent un-cleared take off rolls.
Even better, a light as visual reference would be useful for
all in range. The best situation would be for the aircraft to
transmit the status to ATC to highlight any disagree status
with the controller.
*A runway is preferable. It's considered poor form to be
landing on a taxi way, Harrison.
** Or whatever max reasonable taxi thrust might be for the
type, for a Cessna 172 in mud, that might be 127%.
||//that phrase "take-off" is a problem.//
||I agree, the natural progression is to remove from the cockpit
anyone named "Charlie" or "Juliet" to remove the possibility of
confusion. However, my friend Fueldump Geardown disagrees
||[bs0u0155]; along with Mike, Oscar, anyone who is a father
(Papa) & Romeo?
||// Mike, Oscar, anyone who is a father (Papa) & Romeo?//
||Yep. Gone. Safety you see, can't argue with safety. Trust me
||Take off already eh... hosers.
||//Take Off Clearance Switch//
||OK, I've done that. What should I do with this dangling wire?
||Based on your logic, it seems very strange that we allow pilots, who
vary hugely in experience, intelligence, risk-appetite,
sleep-deprivation, etc. to fly aeroplanes at all in the busy, complex
environment of an airport. Surely it would be far better to have
aeroplanes flown by remote control from the air traffic control tower
until theyre a few miles away from the airport?
||Even better would be to divert the aeroplanes away from the airport and get them to land and take off somewhere else, somewhere quieter with fewer other distractions around.
||You had me until you linked it to the avionics. That would add a number of points of failure. What happens when the computer decides you didn't actually have clearance and cuts your power halfway through the takeoff roll?
||What's needed is a system equivalent to the metal
keys, used on the railways to ensure that trains
could not enter shared lines, as the key was
needed to open the points, and there was only one
key. This made it impossible for two trains to have
it at once.
||<marketing department>//a number of points of failure// -
this is just another way of saying "points of success".
||// What's needed is a system equivalent to the metal keys, used on the railways to ensure that trains could not enter shared lines, as the key was needed to open the points, and there was only one key. This made it impossible for two trains to have it at once.//
||This is actually possible. Put a light in the cockpit and pass a cryptographically signed (and timestamped) key the hardware uses to turn on the light. No light, no permission. It can use a wireless network for the airplanes to pass it about. Contains one button to request the key and one to pass it on. Is automatically passed when the signal timing indicates an airplane has departed.The control tower has the authority to revoke and re-issue when needed.
||//...What happens when the computer decides you didn't
actually have clearance and cuts your power halfway
through the takeoff roll?//
||This is just how it should be implemented. I suggested the
idea that say ~30% of total thrust be the maximum unless
the clearance switch is engaged. If we think what would
happen in that scenario, it all ends quite safely I think.
Say we have made it to the end of runway, we have run
the before take off checklist Pilot Flying (PF) advances
thrust levers with the brake engaged.
||Now, in a lot of aircraft, the engines are spooled up to
40% N1 and allowed to stabilize before advancing further,
to prevent any asymmetry. If we had a clearance switch
interlock here, either the thrust levers wouldn't go to 40%
or the engines would stabilize at 30% and we'd have a big
no-go red flag. Say the clearance switch limited to 50%*,
we stabilize just fine at 40% N1 at the end of the runway
and then advance to 100%. In this scenario again, either
the thrust levers don't go there, or, the aircraft begins
barreling down the runway accelerating far too slowly.
Pilot Monitoring, while calling out the speeds should
notice the lack of acceleration rate, PF, should notice
too. Both would definitely notice that the thrust levers
are firewalled while the engines level out at 50%. Again,
big red flag. Abort take off roll, bring to a stop safely.
big benefit being that the aircraft was never in a high
enough energy situation to fly. The worst case scenario is
not noticing the take off roll is going on forever and
crashing through the lights/antennas off the end of the
runway at an even 80 knots, but hopefully some kind of
alarm should be going off well before that.
*again, I don't know how much thrust is required for every
conceivable taxi situation, I imagine heavy aircraft sitting
on hot tarmac might need some significant unsticking on
||It would be safer if aeroplanes took off really slowly