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Starting engine with fan rotating backwards

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Starting engine with fan rotating backwards

Old 21st Mar 2020, 15:07
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Starting engine with fan rotating backwards

I had not actually thought of it before. Yes I have seen the fan moving the opposite direction from the airport terminal window but it seems that it is OK that the lightoff happens even while the fan is spinning the wrong way. The N speed will be indicating, decrease to zero and then increase again.

This is for the CFM-56...not sure about other types but I have seen maximum allowable speeds for starting."

"Prior to starting, tailwinds can cause the fan to rotate in a direction
opposite to this normal operating direction.
A strong tailwind condition may result in a positive fan speed
indication prior to engine start even though the fan is rotating in a
direction opposite to the direction of normal rotation. To minimize
starter operating time, it is permitted to introduce fuel and ignition
with the fan rotating in a counter-clockwise direction.
During the engine start procedure, airflow through the core engine
will override the tailwind effect and gradually turn the fan in the
correct direction. Once the start is initiated, the fan speed indication
will decrease to zero and then increase as the fan speed increases in
the correct direction. To verify correct fan rotation, monitor the fan
speed (N1) indication prior to and during the start."
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Old 21st Mar 2020, 15:49
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I can neither see nor have an indication of whether and which direction the fan is spinning when I hit the start button, as there is a cut-off that prevents low N1 values from being indicated. Not a problem. (CF34)
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Old 21st Mar 2020, 16:05
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Years ago an aircraft type I flew had a restriction for a minimum N1 (5%???) before adding fuel with a tailwind. RB211's???
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Old 21st Mar 2020, 16:47
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Even more years ago the mechanic would hand signal or headset call rotation when he saw the fan rotate, back in 1974 a BOAC flight was cancelled due the crew had no N2 reading, they missed the fact that for the N1 to rotate the N2 had to be good. Whether it is a twin spool or triple positive N1 rotation should be a good practice; of course all is left to the FADEC on modern jets.
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Old 21st Mar 2020, 16:56
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B737 CFM56, you donít add the fuel unless you have positive N1 rotation.

it only takes a small amount of SA to work out if itís spinning backwards prior to the starter engagement, then you wait for it to wind down to zero, then count back up again before adding the fuel 👍🏼
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Old 21st Mar 2020, 17:29
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Originally Posted by Switchbait View Post
B737 CFM56, you donít add the fuel unless you have positive N1 rotation.
Does a positive N1 indication say the rotor is spinning in the right direction?
With other words what does the N1 indicator show if the fan is rotating backwards?

Just asking
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Old 21st Mar 2020, 17:36
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Being old school, I still give a warning to the flight crew during pushback if we have tail wind. The similarly old school captains always say thank you and occasionally ask for a positive N1 call out.
The younger generation normally just reply with a puzzled "I'm sorry could you repeat that please"?
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Old 21st Mar 2020, 18:00
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tcasblue's post above covers that. The N1 indicates whichever way the rotation is! Even if the sense is wrong. With a warm engine it is not unusual to see the EGT increasing initially with N2 rotation by the starter as the correct direction of airflow through the engine is started - this is in contrast to the usual indication of decreasing EGT with N2 spinup.
Note - this applies to the CFM56

Last edited by Meikleour; 21st Mar 2020 at 18:04. Reason: note added
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Old 21st Mar 2020, 18:23
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Originally Posted by Meikleour View Post
tcasblue's post above covers that. The N1 indicates whichever way the rotation is!
Indeed, disregard my previous question.
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Old 22nd Mar 2020, 03:18
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On the B737-CFM56 you need a minimum of 20% N2 to open the fuel (Start Levers), during a normal start these are opened at 25% N2.
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Old 22nd Mar 2020, 10:13
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If I recall, on the Bristol Britannia, once prop brakes were released the ground crew would hold a prop blade in a tailwind until the starter pulled it out if their hand.
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Old 22nd Mar 2020, 20:14
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Originally Posted by misd-agin View Post
Years ago an aircraft type I flew had a restriction for a minimum N1 (5%???) before adding fuel with a tailwind. RB211's???
This morning the brain's going "maybe it was a minimum N2/N3? X recommended, Y required with a tailwind??" The engines were a lot more finicky before FADEC's. Older Captains told me the early high bypass engines were even more prone to over-temping while starting.
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Old 22nd Mar 2020, 22:53
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The SEPECAT Jaguar had a "correct rotation" light in the cockpit for each engine. You had to see the light lit before advancing the throttle to ground idle on start up. If there was a strong tail wind, the groundcrew would often hold a dustbin lid to the jetpipe until the pilot initiated the start.
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Old 23rd Mar 2020, 00:43
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Re Bristol Britannia, yes, held props in my apprenticeship years (long ago) but being a free turbine engine it was the airflow through the engine as the starter wound up the core that caused the prop to start to rotate.
Many years later when handling BA747-136's out in the Far East I was delighted one day to see BA's newest 747 and the first with JT9D-7A engines, taxi up to the gate. On departure I had just called "N1 Rotation" to the crew during the first engine start when the fan came to a shuddering halt. I wondered "what the ???" but decided to wait until light off before saying anything. As soon as there was a belch of heat out the back the fan suddenly started rotating again and quickly accelerated to normal speed. Only 1 engine did this. A few days later a newsletter from BA Engineering arrived saying this sometimes happened with new -7A engines and was caused by the LP turbine case cooling and shrinking on to the LP turbine blades as the engine motored on the starter bringing N1 to a halt. Light off immediately expanded the case and freed N1. Phew, made the right call that time but never experienced it again!

Early 747's with JT9's were sometimes difficult to start with a tailwind and on occasions an aircraft had to be positioned more or less into wind to get a temperamental engine to start. The tailwind tended to cause a hot or hung start.
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