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Friday, February 17, 2012

Trim Runs - A Tale of the Wrench

I kneel under the fuselage of the DC-8
The anti-collision beacon rotating slowly three feet in front of me
Illuminating my face in red flashes
The dimness of the summer dusk all around

Four forty year old Pratt & Whitney JT3D jet engines
Two on my left
Two on my right
Idle noisily, a dull rumble heard through my earplugs
The ground man
Stationed thirty yards in front of and to the captain's side of the cockpit
Watches the aircraft, the man in the captain's seat, and me

The ground man's hands suddenly point at the aircraft
His right hand points straight up with the index finger
Making a tight spinning motion
Almost simultaneously all four engines spool up
A quartet of rotating internal combustion
Running up to full military power
The roaring of the engines permeates me
I see the nose gear strut slowly compress as the power builds
The aircraft straining against its brakes and chocks
Glancing behind me
Seeing the cloud of black smoke pouring out behind the engines
My vision blurring slightly from the pounding of the sound waves crashing into me

For one minute
The world outside stops

All that exists is this little bubble of concrete
The airplane
Me
And the ground man
All of us alert to the aircraft
It's vibrating
It's rocking
Wingtips buffeting
Wary of a breakaway
Ready to get the hell out of the way

Then it's over
The engines scream back down to idle
The relative quiet an odd feel
It's amazing what you can grow accustomed to
Become a part of

I get the signal from the ground man
Who just got the same signal from the man in the captains seat

Showing me two fingers like a peace sign
He then points upwards with his finger
Touching his cheek below his right eye
And gives me a thumbs up
Then flashes me all ten fingers
Then six fingers

A pause

Again the two finger peace sign
Then he salutes me like I'm a general
And gives me a thumbs down
Then flashes nine fingers

I know what that means

Number two engine: idle ("eye"-dle): adjust up: sixteen clicks

Number two engine: military ("salute"): adjust down: nine clicks


I pull my 1/8" hex key out of my pocket
As I walk to the number two engine
Idling on the left wing
I get down and crawl under the middle of the engine
Peering up into the cowling through a removed access panel
I see the two adjustment set screws

Cast into the housing of the fuel control unit
Next to each screw are letters

One says "IDLE"
The other "MIL"

Inserting my key into the idle screw I turn it clockwise
Feeling the clicks even above the vibration of the idling engine
Counting carefully until I get to sixteen

Switching to the military adjustment screw
I carefully start turning it counterclockwise
I count carefully all the clicks
Going all the way to nineteen
Then reversing
And counting back up ten clicks

It's a little spring loaded valve that gets adjusted in or out with the screws
When going up, or clockwise
The little guy is simply compressed
Opening the orifice slightly

When going down, or counter clockwise
The spring is relieved and to insure an accurate setting
You take it ten clicks past where you want to be
And finish in the more accurate tightening direction

Old school science

Turning towards the ground man
He points to his eye and gives me a nod
The idle adjusted in perfectly
Now to check the military setting once again

I walk back to my spot under the belly of the airplane
The red anti-collision light once again flashing near me
A raised finger
Spinning up
The engines roar to life once more
The sound waves buffet me pleasantly
I close my eyes for a few seconds
And lose myself in the moment




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