737 Checklist question re use of Start switches
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737 Checklist question re use of Start switches
Start Switches.
Before Boeing embarked on a major change to checklist design several years ago and amplified areas of responsibility, the start switches were only set to Continuous during line up. Probably for bird strike precaution. Nowadays the start switches are turned to CONT before taxi and remain on until the after take off checks and then at pilot discretion. After all, the reason for start switches to CONT is for icing conditions and bird strike precaution yet they are left to CONT during taxi even in good weather.
The operating handbook for the JT8D engine cautioned against unnecessary use of the start switches because of a finite time life of the ignitors. Where engine anti-ice was used in flight, the start switches were first switched to CONT. Then engine anti-ice switched on. Then start switches turned off. Nowadays if engine anti-ice is required the start switches are left on for the period the engine anti-ice is on. Why is this so when it wasn't the policy before on the JT8D engine?
Currently the start switches are selected to CONT as part of the landing checks which are usually completed a minute or so before landing. It was like that on the 737-200 as well as the Classics and NG.
Question: If ignitor life was a factor in leaving the start switches until line up in the JT8D engines on the 737-200/ DC9, then is ignitor life not a factor anymore on the 737 Classics and NG series? And why are the start switches left until the last minute during the landing checks when it would be more simple to put the start switches to CONT at (say) 10,000 ft and thus reduce the landing checklist by one item? Assuming that on the CFM 56 series ignitor life is unlimited.
Before Boeing embarked on a major change to checklist design several years ago and amplified areas of responsibility, the start switches were only set to Continuous during line up. Probably for bird strike precaution. Nowadays the start switches are turned to CONT before taxi and remain on until the after take off checks and then at pilot discretion. After all, the reason for start switches to CONT is for icing conditions and bird strike precaution yet they are left to CONT during taxi even in good weather.
The operating handbook for the JT8D engine cautioned against unnecessary use of the start switches because of a finite time life of the ignitors. Where engine anti-ice was used in flight, the start switches were first switched to CONT. Then engine anti-ice switched on. Then start switches turned off. Nowadays if engine anti-ice is required the start switches are left on for the period the engine anti-ice is on. Why is this so when it wasn't the policy before on the JT8D engine?
Currently the start switches are selected to CONT as part of the landing checks which are usually completed a minute or so before landing. It was like that on the 737-200 as well as the Classics and NG.
Question: If ignitor life was a factor in leaving the start switches until line up in the JT8D engines on the 737-200/ DC9, then is ignitor life not a factor anymore on the 737 Classics and NG series? And why are the start switches left until the last minute during the landing checks when it would be more simple to put the start switches to CONT at (say) 10,000 ft and thus reduce the landing checklist by one item? Assuming that on the CFM 56 series ignitor life is unlimited.
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Apples and oranges? What does the JT8 have to do with the CFM56? Of course the ignitors have a limited life, but the CFM56 is a lot more susceptible to water ingestion (especially during ground ops since they are hanging only a few inches over the ground on the 737s). There have been numerous flameout events in the CFM56 in heavy rain as well, so keeping them on during engine anti ice operation is just an extra precaution to keep the relatively (compared to the JT8) small core running going...
Last edited by STBYRUD; 29th Oct 2014 at 16:40.
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The manufacturer here certainly wouldn't care about life of the ignites in any case.They would only care about their serviceability, normal abnormal and backup!!
To hell with every thing else
To hell with every thing else
CFM engines on A320 used to select IGNITION on for take-off & landing too, but it is no longer SOP (unless required for icing), whereas IAE engines on A320 will auto-select IGNITION on for approach - as I understand it at least.
This would suggest it is a P & W versus CFM operational philosophy thing.
This would suggest it is a P & W versus CFM operational philosophy thing.
I believe that it is nothing to do with the prospect that engines might flame out during taxi, or even during normal takeoff or go around. Any engine that was inclined to do that would be unlikely to gain certification. Early CFM installations did suffer flame outs and roll back from water ingestion but that was fixed 20 years ago.
The requirement to use ignition on takeoff and approach probably only remains as an additional layer of protection against compressor stall, water ingestion or bird strike.
The reason for moving ignition from when it was originally done at line up to the after start checks came in when Boeing rightly decided to cut down checklists when the aircraft was on the move. Boeing always was nicely brief, but now their taxi checks are absolutely pared to the bone.
But good point, selecting igniters on descent would continue the philosophy of reducing the reading of 'stuff' in checklists at busy times. Maybe it should be tied in with resetting altimeters passing transition?
The requirement to use ignition on takeoff and approach probably only remains as an additional layer of protection against compressor stall, water ingestion or bird strike.
The reason for moving ignition from when it was originally done at line up to the after start checks came in when Boeing rightly decided to cut down checklists when the aircraft was on the move. Boeing always was nicely brief, but now their taxi checks are absolutely pared to the bone.
But good point, selecting igniters on descent would continue the philosophy of reducing the reading of 'stuff' in checklists at busy times. Maybe it should be tied in with resetting altimeters passing transition?
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The newer NG's have the auto function found on the larger Boeings. Ignition is automatically activated when engine anti-ice is selected or when flaps are extended & turned off when engine anti-ice is turned off or flaps are retracted. This means they are activated at approximately the same time as they are when done manually on aircraft without the auto function.
