Dear friends and followers, welcome back to my channel
and to a great video you guys have requested:
Why do you sometimes see planes taxiing around the apron
with only one engine running?
We'll be looking at the primary reasons why pilots do so
and go through the procedures that exist on different types of planes.
So, lets get started!
[Air Traffic Control conversation]
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I'm sure you have spotted a turboprop Dash Q400 or the Airbus A320
taxiing towards the gate with only 1 engine running.
Now, I'm not kidding, I've seen passengers getting worried that they're about to board
a plane which arrived with an inoperative engine.
Well, to calm your nerves, the engine was working fine until the
pilots purposely turned one of them off.
But, "why?" is the question today.
Reason number three: noise.
Airplane engines and the Auxiliary Power Unit
contribute so much noise near an airport that
every shut-downed engine helps.
As a passenger you can't really hear
the difference if one of the engines
has been shut down during taxi. Okay, maybe if you're sitting in the far-end
of an MD-80.
But, it sure makes the difference for the nearby residential areas and the
ground crews. As the airplane approaches the gate position.
Reason number 2:
Engine wear.
Ok, the reduction in engine wear is minor
as the engine is only shut down for a few minutes after landing
and on most planes you can only shut down one
specific engine due to the system layout of the plane
but I'll come back to that in a minute.
Now, maintenance reports state
that a lot of the engines get damaged due to
bits of gravel being
sucked up during taxi or so-called "FOD";
Foreign Object Damage.
So, shutting down an engine earlier reduces the risk of
potential FOD damage.
But the primary reason as every so often is
fuel saving. Now to give you an example
a Boeing 737-800 burns
approximately 300 kilos
of fuel per hour with engines running
at idle power. Now, let's say that you just landed
and are stuck in inbound taxiing traffic
for thirty minutes. You could cut
engine number two and you could save 75 kilograms
of fuel that equals two
petrol tanks of a standard size car.
Now, let's use Alaska Airlines as an example.
They have 160 Boeing 737s
in their fleet. Now, let's say each
have an average taxi time of fifteen minutes
with one engine shut down
equals 37.5 kilograms
of fuel times
160 planes equals
six tons. But, each plane
performs at least two flights per day,
so the total fuel saved would be twelve tons.
That's a lot of fuel for one day,
multiply that by 365 days per year,
and you are saving a LOT of money.
So it does make sense, doesn't it?
But, how is it done? Can you just turn off
any engine whenever you want to after landing?
Or even taxi out with one engine shut down
and start it up just prior takeoff?
To answer this question I got two of
my good pilot friends helping me out. Let's see how
my dear friend and Canadian colleague pilot Emilie
performs the procedure on the Dash Q400. Emilie,
Emilie: What is going on, you guys
my name is Emilie and my good friend Joe
had asked me to film a little segment and get something that he's been
getting a lot of questions about, is
"Why do certain Q400s
or Dash-8s, why do they taxi with
a single engine on?" So just having one of their propellors spinning.
When we're starting to pushback,
we only really need one engine to actually power up the entire aircraft
so once we've disconnected from the ground power, or we've
switched over from having our APU so our third
turbine that's in the back, in the tail, well so once we've
switched over to our engines being operative,
we can actually power the entire cabin and the
airflow system, we can basically power everything up with just one
engine, so basically just to keep fuel costs down,
pushback, taxi out with a single engine and then once we're getting
you know, further down the taxiway,
we'll start up our second one so to do that, we'll essentially
just use our
start-select switch, select the proper engine,
so once it takes over, essentially, we have our secondary engine
powered up, and we're ready for departure. So that's why you guys see us
very often with just that single propeller spinning
it's really just to save costs.
So, I hope that that helps as a little explanation for you guys, um, I hope you guys enjoyed my little tid-bit,
and hopefully Joe can help you with the rest of your questions,
so, thank you so much guys.
Joe: Thank you very much, Emilie, for that great explanation and I have another
one; pilot an YouTuber, Swayne Martin,
will show us how the procedure is done on the
Embraer 145. Swayne, off you go.
Swayne: Hey guys, my name is Swayne Martin and I'm a First Officer here
at Envoy Air on the Embraer 145, and here at Envoy, we're
generally cleared to start our number one engine
on pushback from the gate. That's for fuel savings, and
we leave only that number one engine running all the way until takeoff,
or just a few minutes beforehand, at which point we start our number two
engine, giving it to warm up and get started before takeoff.
The opposite is true on arrival.
When I land and I'm taxiing to the gate, often we shut down our
number one engine after its cool-down period of sixty seconds
and we taxi all the way to the gate on just the number two engine.
That's for one big reason; when we pull into the gate,
our baggage handlers have to approach the airplane and the
baggage compartment which is right underneath the number one engine
to start unloading bags, and they can't do that until
the number one engine is off, so that's often why we'll often leave the
number two engine running on the other side of the airplane,
and go ahead and shut down the number one engine.
Joe: Thank you very much, guys for your help with this video. Really appreciate it!
Now, on the 747,
400 or Dash-8, we cut
engine number three after landing.
Now, you usually ask, "Why engine three,
and not one and four?"
Now, if we look at this hydraulic synoptic of this 747
in more detail, you'll quickly see why.
Now, whilst the engines are running, each engine
pressurizes its respective hydraulic
system via the engine-driven pump.
We'll neglect the demand pumps for a second.
Now, if we look at the different systems powered by
hydraulic system one, you'll see two
primary systems we'll be needing
to taxi the 747.
The nose steering actuation and the body gear steering actuation.
Secondary is the alternate braking system which
is also powered by
hydraulic system two. So, cutting off
engine number two wouldn't be very beneficial
in case the alternate braking system is
suddenly needed.
Now, if we look at hydraulic system number four,
the most important system here
for taxiing is the normal brakes system.
So, turning off engine four is the last thing
you want to do. And, if we look at system three,
Well,
there's absolutely nothing essential here
for taxiing the 747. Now, coming back
to the demand pumps, now, each system has
a hydraulic demand pump powered
pneumatically or electrically. Now, if the respective
demand pump selector is in the
"auto" position, the demand pump
automatically kicks in, providing the
necessary system pressure in case the
engine-driven pump fails or the fuel
control switch is placed to the
"cut off" position. So, technically speaking,
as we cut off engine number three,
hydraulic system number three is still
powered by its respective demand pump.
So, you might ask, "Why not also turn off
engine number two as the demand pump
would power the alternate brakes system?"
Yes, that's true, and an option as some companies
apply the two-engine-taxi procedure.
But, there are two reasons for making this procedure
fairly dangerous. First,
Taxiing only with the outboard engines running
with a heavily loaded 747 would request
a lot of so-called "breakaway
thrust" to initially
get the plane moving. Now, that there is
a huge risk of foreign object damage,
as the outboard engines often are
beyond or near the end of the paved taxiway.
Also, the breakaway thrust is
so high that the jet blast could
pick up some debris and damage nearby planes.
Plus, the higher thrust output actually annihilates
any fuel savings.
So, as we taxi toward the gate position after landing,
we let the engines cool down for three minutes
and then after conformation
with the captain, you place the fuel control switch
number three to the cutoff position.
and reduce the overall fuel consumption
by one ton per hour. Now, you might have realized that
I primarily spoke about
performing this procedure during taxiing after
landing. Now, what about
taxiing towards the runway and waiting in a
long line of departing airplanes?
Now you could apply this
same procedure, but this primarily depends on
the airplane and airline you work for,
and there are a few factors that do not encourage you to do so.
First, most jet airliners start
their engines under the supervision of the ground crew
as they monitor the start sequence to spot any
fluid leaks or tailpipe fires etc.
to immediately notify the pilots via headset.
Second, jet engines, depending on the make,
need certain warming up times before
applying takeoff thrust to reduce thermal stress.
So, things could get hectic on the taxiway
if the ATC controller advises you to
takeoff from an intersection, and you're sitting there
with only one engine running
and the other isn't even started nor warmed up yet.
Not the best idea [chuckles].
I hope this explanation helped a little to get
a better understanding of why and
which engine pilots shut off during taxi
to save fuel, the environment, and reduce
the overall noise level at airports.
That's it for today, thank you very much for your time,
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And don't forget, a good pilot is always learning,
See you next week, wishing you all the best, your Captain Joe.