GE CF6 Question
 

As many of you know once in a while a question comes up that seems self-evident but is not covered in the “book”. We operate the B-747 equipped with the GE CF6-50E2 and place the start lever to idle at 15% N2. Is there any problem associated with placing the start lever to idle late, i.e. 18% or 20% or max motor? The best example that comes to mind is during a start abort after motoring to lower EGT for a new start attempt.

CF6
 

As far as I am aware the reason for CF6's fuel to be placed in early is that the fuel is the lubricant for the Fuel control unit and needs a fuel flow to lubricate properly or the unit damages itself.

As far as I know!!! :ok:

Spanner

The cf6 is a hydro mechanical engine-fuel being the hydro bit.If I could be bothered to get my fcom out I could bore you for hours about it!The engine always seems to run hot compared to a rolls or IAE.Putting the fuel in later should keep the egt down a little on start-it depends whether you have fadec looking after it or just a pmc.

The GE CF6-50E2 is purely mechanical. I'm with flyingspanner there. From what I remember the HMU and pump reqd. the fuel for lubrication purposes.
The motoring over after a hot start is the lesser of two evils, one bringing the EGT down to an acceptable level for a restart attempt, and two, a bit of premature wear to the mechanicals.

I was thinking of the cf6-80c2a5,where do they get all those numbers from??

Here’s where they get all those numbers from !!

MODEL APPLICATION

A1 A300-600, MD11

A2 A310-300, MD11

A3 A300-600

A4 A310-200

B1 747-200, 747-300, 747-400

B2/B3 767-200, 767-300

B4/B6 767-300ER


The CF6-80C2 series Engines are basically the same physically.
However the -B1(A1) through to -B6(A6) rating is a product of different EEC
programs and MEC cams.

The CF6-50 series engine is the predecessor to the –80 and for all intents is basically the same in operation, the main difference being the location of the accessory gearbox, which you’ll find on the fancase whereas on the -80 the gearbox has been moved to the core, this reduced the external profile/diameter of the engine assy (required for ground clearance during the design of the 767 and various Airbuses).

As for the ‘Fuel On’ question, FlyingSpanner and Gas Path are correct in a purely Technical sense – yes the pump is fuel lubed and the Maint Manual does say to ensure that positive fuel press is supplied to the pump inlet during eng motoring. Below is from Maint Manual;

CAUTION
The main engine fuel pump and main engine control are
fuel lubricated. Do not motor engine unless a positive fuel
inlet pressure is indicated.
CAUTION

ZONER asked:
Is there any problem associated with placing the start lever to idle late, i.e. 18% or 20% or max motor? The best example that comes to mind is during a start abort after motoring to lower EGT for a new start attempt.

Well in reality now, NO there is no problem placing the start lever to run late – in fact it is a lot better for the engines long term health and during Maintenance Engine runs, our procedure is to ensure MAX MOTORING speed before lifting the lever. The faster she is spinning, the more airflow there is through the engine to provide cooling, and the fuel pump outlet pressure will be a lot higher so will provide a better fuel spray pattern from the fuel nozzles. During normal day to day operations, there is always fuel ‘trapped’ in the engine fuel system between the ‘spar’ valve and the fuel control unit so there is your lubrication fuel. The risk of damage to a fuel pump due lack of lube really only presents itself during maintenance e.g a fuel filter/MEC change where the fuel system is drained of all fuel and subsequently an extended Motoring takes place with the system ‘Dry’.


As for the engagement of the starter the following should be a good guide:

Limitations
-The Normal Duty Cycle is 1 minute on followed by 30 seconds
off, indefinitely.
-The Extended Duty Cycle is 5 minutes on followed by 2 1/2
minute cooling period.
-Two consecutive 5 minute cycles require a 10 minute cooling
period between additional 5 minute cycles.

-The recommended re-engagement is 0% N2 rpm. .
- Normal Starter re-engagement is not recommended above 15%
N2 rpm. .
- In case of fire, or to clear fuel, re-engagement is permitted up
to 30% N2 rpm

CAUTION
RE-ENGAGEMENT ABOVE 30 PERCENT N2 RPM MAY
RESULT IN STARTER OR GEARBOX DAMAGE

Note, that it is the ‘re-engagement’ of the starter that can cause carnage, if the starter is engaged say up to 48% and you decide to place the fuel control ‘off’ for a hung/hot start it is okay to keep the engine turning on the starter as it is already engaged – the engine will just slow down to the Max Motoring speed.

Placing the start lever to idle at 15% N2 makes the light off coincide with max motoring (about 23% around 10 secs later). The engine thus has a continuous acceleration throughout the start.
I realise this doesn't actually answer the Q, but it suggests that GE, being the sensible practical types they are, introduced the 15% figure for this reason and not any technical limitation. Just my 2 cents worth.

Isn't the 15% the minimum req?

Standard maint procs seem to be to motor to max rpm. Also some req that the spar (LP) valve is maintained open during dry motoring to ensure a positive fuel supply to the moving parts EG HMU, pump.:ok:

The CF6-50C family in the DC-10 had to live with reduced starter pneumatic pressure due to APU derating, but still started fine with fuel on at 10% N2. See if you can obtain GE's Specific Operating Instructions to be sure. The -50E core is identical to the -50C.

I believe the 15% requirement in the 747 was carryover (commonality) from early JT9's, which genuinely required 15% or more. Several operators had mixed fleets (GE & PW) and Boeing wanted to maintain a common procedure.

I'm getting smarter all the time, thanks for the answers!

Smart people
 

We put the fuel in at 10% N2 on our CF6-50E2's. Never have a problem. Although I would agree with what is said above about no harm in motoring the engine. Especially at High Altitudes or temps.