B747-400 Starter Duty Time Limits (CF6-80)

[cf6-80c2b5f]

From my company manual:

Starter Operating Duty Time
Operation limits of the engine starter:
Normal duty cycle:
• One minute on, one minute off.
Extended duty cycle:
• One to five minutes ON, OFF until 0% N2.
• Five to ten minutes ON, 20 minutes off.
• Ten to fifteen minutes ON, 30 minutes off.

What does Boeing mean by "extended duty cycle"? Is it just arbitrarily used when you (or the autostart) have to exceed the "normal duty cycle" or is some special inspection required after using the extended duty cycle?

For instance, the autostart will attempt three hot starts, motoring 30 seconds between each start. This is clearly above the one-minute normal duty cycle. So, is the normal duty cycle a limit without any meaning?

Thanks.

[JammedStab]

That's odd. We had 5 minutes of continuous operation allowing for the three attempts. Then 30 seconds off for each minute of operation. Some other limits as well.

[cf6-80c2b5f]

That's much easier to understand. Maybe Boeing realized that and decided to change it to something much more difficult, so that no one would know what the true limits are. They seem to have a special affinity for publishing material that is vague and subject to multiple interpretations.

[NSEU]

Your book aligns with what Boeing wrote in their Maintenance Manual for the CF6 (a long time ago). It does seems contradictory.

The Boeing Maintenance Manual for the RB211 specifies a normal maximum of 3 minutes letting the N3 return to zero, then a second 3 minute start cycle, then a 1 minute cycle (Then a 30 minute break).
An extended cycle consists of a continuous 5 minute cycle with a 30 minute break.

Perhaps normal implies "ignition". Usually extended starter duty cyles are used for (among other reasons)
1) warming up oil (prior to fuel/ignition)
2) cooling down engine internals after a flight (so that engineers can climb into the back of the engine for maintenance purposes)
3) post-maintenance, for making sure any pooled fuel in the engine is blown out the back, prior to a normal start.

[NSEU]

P.S. The Boeing Maintenance Manual for the 747-400 PW4000 has some clues but still confuses:

QUOTE:

"The starter cycle limits are in the steps that follow:
(a) The normal duty cycle is two starts, which were tried one after
the other, followed by some time to continuously motor the
engine to decrease the temperature. After the second time the
start and motor was done, you must disengage the starter and
let the N2 decrease to zero before you engage the starter
again.

NOTE: If you motor the engine before you do the starter duty
cycle, it is not necessary to disengage the starter
before you try to start the engine

CAUTION: Do not engage the starter at a speed more than 15%N2. If
you do this, damage to the starter can occur.

(b) If the starter is not engaged, it is recommended that you do
not engage the starter at a speed of more than 15%N2.

NOTE: The speed of 15%N2 is recommended to decrease the
quantity of worn areas of the starter.

(c) If an engine start is stopped (aborted) at more than 48% N2,
you must let N2 decrease to less than 5% N2 before you try the
next start.

NOTE: This will remove power from the EEC and reset the EEC
overspeed protection logic.

(d) The starter duty cycles which are extended and the cooling
limits to motor the engine (with the fuel and ignition OFF and
the FUEL CONTROL switch in the CUTOFF position) are as follows:
1) 0~5 minutes ON - Disengage the starter and let the N2
speed decrease to zero before you engage the starter again.
2) 5~10 minutes maximum ON: Stop for 20-minutes to let the starter
decrease in temperature.
3) 10~15 minutes maximum ON: Stop for 30-minutes to let the
starter decrease in temperature.
(e) You can motor the engine at speeds as low as 10% N2 rpm for a
maximum of two minutes."

[tdracer]

From the GE CF6-80C2 Operating Instructions:


B. Duty Cycle.
(1) Starter Maximum Continuous Operation is 5 minutes
(2) Cool starter 30 seconds per minute of operation.
(3) After two consecutive 5-minute duty cycles, cool starter for 10 minutes prior to each subsequent 5-minute duty cycle.


The AMM words are not intended for 'normal' day to day engine starts - they're intended to apply to specific AMM tasks such as wet/dry motoring, leak checks, actuator tests, etc.

[cf6-80c2b5f]

Thanks to all three of you guys for responding.

Tdracer, what you posted above seems to be right in line with JammedStab's post and it definitely seems easier to remember than what's in my company's AFM.

Unfortunately, looking at my GE CF6-80 AMM from 2007, it seems to follow my company's AFM limits and closer to NSEU's post for the P&W:

The operation limits of the engine starter are as follows:
1) The normal duty cycle is one (1) minute on followed by one (1) minute off.
2) The extended duty cycle and cool down time are as follows:
a) Zero (0) to five (5) minutes ON, followed by power OFF until 0% N2 is reached or,
b) Five (5) to ten (10) minutes ON followed by 20 minutes OFF or,
c) Ten (10) to fifteen (15) minutes ON, followed by 30 minutes OFF.So, I guess it's safe to assume you can do 5 minutes on and ignore the normal duty cycle. Otherwise, you couldn't attempt more than one or two starts.