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Faro
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Why Fuel Control Switches Held During Start?

Tue Oct 20, 2009 10:24 am

Often in videos of engine start procedures, the fuel control switches are held -almost pushed in fact- manually by the pilots for a certain time. Why is this? Are they spring-loaded and need to be kept in place until the engine start is achieved satisfactorily? If so why?

Faro
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fr8mech
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RE: Why Fuel Control Switches Held During Start?

Tue Oct 20, 2009 11:30 am

The pilot or mechanic will 'gaurd' the switch. This is to ensure that, in the event of an abnormal start (usually a hot start) he can quickly move the switch to cut-off. He will usually move his hand away after starter cut-out.
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zeke
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RE: Why Fuel Control Switches Held During Start?

Tue Oct 20, 2009 12:56 pm



Quoting Faro (Thread starter):
Often in videos of engine start procedures, the fuel control switches are held -almost pushed in fact- manually by the pilots for a certain time. Why is this? Are they spring-loaded and need to be kept in place until the engine start is achieved satisfactorily? If so why?

That is not true for any of the FBW Airbus aircraft, for a normal start they are not guarded.
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CosmicCruiser
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RE: Why Fuel Control Switches Held During Start?

Tue Oct 20, 2009 3:21 pm



Quoting Zeke (Reply 2):
That is not true for any of the FBW Airbus aircraft, for a normal start they are not guarded.

I don't fly our Airbus but wouldn't that be more a co. policy? We've guarded the fuel switches on all the jets I've ever flown.
 
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zeke
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RE: Why Fuel Control Switches Held During Start?

Tue Oct 20, 2009 4:03 pm



Quoting CosmicCruiser (Reply 3):
I don't fly our Airbus but wouldn't that be more a co. policy? We've guarded the fuel switches on all the jets I've ever flown.

All the FBW aircraft have FADEC controlled start sequences, Airbus says not to accidentally turn off the engine master switch as that would interrupt the FADEC automatic start sequence (e.g. motoring after a hot start). AFAIK it is not a "company" policy, rather Airbus SOP.
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tdscanuck
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RE: Why Fuel Control Switches Held During Start?

Wed Oct 21, 2009 12:56 am



Quoting Zeke (Reply 4):
All the FBW aircraft have FADEC controlled start sequences, Airbus says not to accidentally turn off the engine master switch as that would interrupt the FADEC automatic start sequence (e.g. motoring after a hot start).

I know quite a few pilots who still guard the start switch on FADEC engines, but it may be force of habit rather than OEM SOP.

Tom.
 
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RE: Why Fuel Control Switches Held During Start?

Wed Oct 21, 2009 1:25 am



Quoting Zeke (Reply 4):
All the FBW aircraft have FADEC controlled start sequences,

As does the MD-11 but we still guard the start switches. there's no guarantee that FADEC will control a bad start otherwise the manufacturer wouldn't have start temp limits that must be observed.
 
fr8mech
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RE: Why Fuel Control Switches Held During Start?

Wed Oct 21, 2009 1:50 am

Quoting Zeke (Reply 4):
All the FBW aircraft have FADEC controlled start sequences, Airbus says not to accidentally turn off the engine master switch as that would interrupt the FADEC automatic start sequence (e.g. motoring after a hot start). AFAIK it is not a "company" policy, rather Airbus SOP.

If a FADEC engine overtemps or hangs, it is likely the result of a faulty EEC. Why would you now trust that same EEC to follow the appropriate protocol and protect the engine?

I was trained to always gaurd the switches/throttles and that was the way I trained it.

[Edited 2009-10-20 18:51:34]
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zeke
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RE: Why Fuel Control Switches Held During Start?

Wed Oct 21, 2009 2:42 am



Quoting Tdscanuck (Reply 5):
I know quite a few pilots who still guard the start switch on FADEC engines, but it may be force of habit rather than OEM SOP.

That would be described as technique rather than procedure.

Quoting CosmicCruiser (Reply 6):

As does the MD-11 but we still guard the start switches. there's no guarantee that FADEC will control a bad start otherwise the manufacturer wouldn't have start temp limits that must be observed.

Airbus also specifies start limits, but FADEC will catch a start problem with an auto start before a pilot would. The procedure we use for an normal autostart, manual start, and air start are different.

Quoting Fr8Mech (Reply 7):
If a FADEC engine overtemps or hangs, it is likely the result of a faulty EEC. Why would you now trust that same EEC to follow the appropriate protocol and protect the engine?

Not true, could be altitude, tailwind up the tailpipe, bleed pressure, old engine etc. Never had a start problem because of a bad EEC.

Quoting Fr8Mech (Reply 7):
I was trained to always gaurd the switches/throttles and that was the way I trained it.

You can do what you want, that is your technique, I must also stress that the procedures used by ground mechanics is not the same as what is used by pilots. The aircraft manufacturers have different procedures for engine ground runs and starting and aircraft for a normal flight. The thread starter was talking about starting engines for a normal flight by pilots, not mechanics.

The other reason why you would guard switches during ground runs is that you often have mechanics looking at the engine while it is running, and you have you hand on the engine master in-case the unthinkable happened to them.
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RE: Why Fuel Control Switches Held During Start?

Wed Oct 21, 2009 5:50 am



Quoting Faro (Thread starter):
manually by the pilots for a certain time. Why is this? Are they spring-loaded and need to be kept in place until the engine start is achieved satisfactorily? If so why?

I don't know of any "requirement" to do this, but it is a good technique that has been taught for ages. Bottom line is that you may need to shutdown the engine quickly and leaving your hand on the lever/switch that you will need to do that just means you can quickly secure the proper engine without making a mistake. It's not like you've got something else to do with your hands at that moment anyway.
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RE: Why Fuel Control Switches Held During Start?

Wed Oct 21, 2009 9:56 am



Quoting Faro (Thread starter):
the fuel control
switches are held -almost pushed in fact- manually by the pilots for a certain time. Why is this?

Look at the question from a slightly different angle.
On some aircraft, powered by RollsRoyce RB.211 engines, the engine fuel control switch has a position called 'enrich', and in that position, which must be held manually by the pilot, extra fuel is metered to the engine, during starts in cold weather, to aid the engine to spool up quicker.
Remembering, of course, that these RollsRoyce engines are of a three shaft design, starts can take longer, than with 2-shaft designs.
When the engine has reached idle RPM, the fuel switch is released to the normal 'run' position.
 
Mender
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RE: Why Fuel Control Switches Held During Start?

Wed Oct 21, 2009 12:37 pm



Quoting Faro (Thread starter):
Why is this? Are they spring-loaded and need to be kept in place until the engine start is achieved satisfactorily? If so why?

To start a Boeing 737, 757, 767 you initiate the start by turning the ignition switch on the overhead panel to Start. This opens the air valve on the starter and turns on the ignition system. Then when the engine is turning at the correct speed you introduce the fuel using the lever below the throttles that you asked about in the original question. This lever is not spring loaded but you do have to lift the switch/lever out of a detent. this prevents it from being accidentally knocked from one position to the other and I agree with reply 1 and 9 as to the main reason they are held during the start procedure.

When you turn the ignition switch on the overhead panel to start it is kind of magnetically held in the start position. Once the engine has reached self sustaining speed (approx 50% dependent on the engine) the magnetic mechanism relaxes and the start switch springs into the auto ignition position.

When the switch is in the auto ignition position the igniters are off unless the engine slows down (flame out), anti ice is turned on or the flaps lowers (ie, landing)

Hope this helps.
 
boeingfixer
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RE: Why Fuel Control Switches Held During Start?

Thu Oct 22, 2009 3:02 pm



Quoting Mender (Reply 11):
To start a Boeing 737, 757, 767 you initiate the start by turning the ignition switch on the overhead panel to Start. This opens the air valve on the starter and turns on the ignition system. Then when the engine is turning at the correct speed you introduce the fuel using the lever below the throttles that you asked about in the original question. This lever is not spring loaded but you do have to lift the switch/lever out of a detent. this prevents it from being accidentally knocked from one position to the other

A couple corrections to the above post.

To be precise the start selector switches on the 737, 757 and 767 don't have a 'start' position and they are not an 'ignition switch'. The 'start' position is called "Ground" (GND) (GRD on the 737). The start selector switch is held in the GND(GRD) position by a solenoid when selected. It 'arms' the ignition system which at this point is not energized and opens the start valve. When the N2 or N3 (RB211) is at the correct speed the Fuel Control Switch is moved from Cut-Off to Run. This energizes the ignition system and opens the engine and spar fuel valves. With normal acceleration the start switch holding solenoid releases at approx. 50% N2 or N3 (RB211)(35% N2 on 737-200) and returns to the Auto (OFF for 737) position.

It's also very good technique to guard the fuel switches during the start regardless of the aircraft. I'm not sure of the airline but I remember a B744 with an RB211 being drastically over-temped during start because the configuration was incorrect for an AUTO start and the fuel switches were not 'guarded'.

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DC8FriendShip
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RE: Why Fuel Control Switches Held During Start?

Thu Oct 22, 2009 11:09 pm

On the CRJ the throttles have a shut off position as opposed to a separate fuel lever. The -700 and -900 are FADEC but even then I still gaurd the throttles on start up untill ITT rollback. only then do I feel safe that the engine is running good.
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Valkyrie01
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RE: Why Fuel Control Switches Held During Start?

Fri Oct 23, 2009 1:25 am

I was speaking to a few mechanics that work on the B747-400 and i remember them telling me the 747-400 has an auto start option, and some of the 747-400 has auto start.
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Mender
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RE: Why Fuel Control Switches Held During Start?

Fri Oct 23, 2009 12:18 pm



Quoting BoeingFixer (Reply 12):
A couple corrections to the above post.

To be precise the start selector switches on the 737, 757 and 767 don't have a 'start' position and they are not an 'ignition switch'. The 'start' position is called "Ground" (GND) (GRD on the 737). The start selector switch is held in the GND(GRD) position by a solenoid when selected. It 'arms' the ignition system which at this point is not energized and opens the start valve. When the N2 or N3 (RB211) is at the correct speed the Fuel Control Switch is moved from Cut-Off to Run. This energizes the ignition system and opens the engine and spar fuel valves. With normal acceleration the start switch holding solenoid releases at approx. 50% N2 or N3 (RB211)(35% N2 on 737-200) and returns to the Auto (OFF for 737) position.

I was writing the above so a lay-man could read and understand. It wasn't my intention to be word perfect, but you are quite right
 
Tristarsteve
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RE: Why Fuel Control Switches Held During Start?

Sat Oct 24, 2009 1:17 pm



Quoting Valkyrie01 (Reply 14):
was speaking to a few mechanics that work on the B747-400 and i remember them telling me the 747-400 has an auto start option, and some of the 747-400 has auto start.

Most, but not all B744 have autostart.
Many years ago a B744 was on a delivery flight. It stopped in CDG to change crew. The new crew got on the brand new aircraft and did an autostart on two engines at the same time. (B744 can start engines two at a time from the APU.) Unfortunately the aircraft did not have autostart. They had done all their training in a simulator with autostart!
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PGNCS
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RE: Why Fuel Control Switches Held During Start?

Sat Oct 24, 2009 6:25 pm



Quoting BoeingFixer (Reply 12):
The 'start' position is called "Ground" (GND) (GRD on the 737). The start selector switch is held in the GND(GRD) position by a solenoid when selected.

As an aside, our 737-200's did not have a solenoid, so the FO had to hold the start selector during the start; the 300's and subsequent had the solenoid.

Quoting BoeingFixer (Reply 12):
It's also very good technique to guard the fuel switches during the start regardless of the aircraft.

I generally agree, especially on tempremental engines like JT8D-200 series (especially with a tailwind); though some aircraft procedures have you not guard the switches so automatic FADEC functionality isn't interrupted like Zeke pointed out.
 
loggat
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RE: Why Fuel Control Switches Held During Start?

Mon Oct 26, 2009 4:08 am

On the E170, the pilot guards the start switches (there are no fuel cutoff switches) and we are instructed to abort the start for a number of reasons even though we probably have the most advanced airplane/FADEC in commercial aviation. The reason that we perform a manual abort on a start is that the FADEC will only command an aborted start if an exceedance is noted (eg. ITT - hot start). By watching the ITT go screaming up towards the red line, we can command an engine start to be aborted and avoid having the engine limitation exceedance. Once the red line has been exceeded, maintenance action is required. If we catch a hot start in progress and abort before reaching red line, we can dry motor, start it again, and be under way with not much ado.

That's my perspective on why we still guard the start/fuel levers.
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tdscanuck
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RE: Why Fuel Control Switches Held During Start?

Mon Oct 26, 2009 5:33 am



Quoting Loggat (Reply 18):
The reason that we perform a manual abort on a start is that the FADEC will only command an aborted start if an exceedance is noted (eg. ITT - hot start). By watching the ITT go screaming up towards the red line, we can command an engine start to be aborted and avoid having the engine limitation exceedance.

Why isn't the FADEC programmed to watch the rate of temperature rise? That seems like a pretty straightforward control algorithm that would prevent an awful lot of damage.

Tom.
 
CX Flyboy
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RE: Why Fuel Control Switches Held During Start?

Mon Oct 26, 2009 2:28 pm

We don't guard the switches on the 777 for the same reasons that Zeke has explained for the Airbus pretty much.
 
boeingfixer
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RE: Why Fuel Control Switches Held During Start?

Wed Oct 28, 2009 6:58 pm



Quoting PGNCS (Reply 17):
As an aside, our 737-200's did not have a solenoid, so the FO had to hold the start selector during the start; the 300's and subsequent had the solenoid.

I've worked on both types on the 737-200's. The one's without the holding solenoid actually had the solenoid but the centrifugal switch in the starter was removed due to reliability purposes. Therefore having to hold the start switch manually with these modified starters was the normal procedure.

Quoting TristarSteve (Reply 16):
Many years ago a B744 was on a delivery flight. It stopped in CDG to change crew. The new crew got on the brand new aircraft and did an autostart on two engines at the same time. (B744 can start engines two at a time from the APU.) Unfortunately the aircraft did not have autostart. They had done all their training in a simulator with autostart!
Two engine changes required.

That was the incident I was thinking about.

Cheers,

John
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RE: Why Fuel Control Switches Held During Start?

Thu Oct 29, 2009 8:27 am



Quoting BoeingFixer (Reply 21):
I've worked on both types on the 737-200's. The one's without the holding solenoid actually had the solenoid but the centrifugal switch in the starter was removed due to reliability purposes. Therefore having to hold the start switch manually with these modified starters was the normal procedure.

Also a faulty Solenoid start swich or unserviceable centrifugal sw in the starter can have the same consequence.
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