Ground Power use
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Ground Power use
Hi All
If starting via a ground power plug is necessary (Cessna 172) then what should be the position of the master switches please?
many thanks
If starting via a ground power plug is necessary (Cessna 172) then what should be the position of the master switches please?
many thanks
Last edited by WorkingHard; 27th Dec 2009 at 11:28.
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ab33t
Are you sure about the master switch?
On most light aircraft the master switch connects the battery to the electrical system via the battery contactor, usualy the appication of ground power automaticly pulls in the ground power contactor to apply ground power to the aircraft systems, this keeps the aircraft battery disconected from the aircraft electrical system.
I could be wrong in the case of the C172 but if so it has an unusual electrical system.
On most light aircraft the master switch connects the battery to the electrical system via the battery contactor, usualy the appication of ground power automaticly pulls in the ground power contactor to apply ground power to the aircraft systems, this keeps the aircraft battery disconected from the aircraft electrical system.
I could be wrong in the case of the C172 but if so it has an unusual electrical system.
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I support A and C's comment. In any case, ground power should be applied as described in the flight manual - which it is for the 172. Generally the master switch is not selected "on" until ground power is removed, but it is very aircraft type specific, so read the flight manual. If you're needing ground power, you're past the point where you are willing to accept yet another problem because something is not done right.
The proposal to use ground power on a 172 "pops up a few little red flags" for me...
Is the ground power necessary because the battery was found to be discharged? Cause found and corrected before flying attempted again? (usually master was left on - but be sure it's not another failure). Could the battery have frozen while discharged? Frozen battery = buy a new battery before flying.
Is the ground power necessary because it was run down trying to start? It should have started, was the reason for not starting found and corrected? If so, has a suitable period of time elapsed (30 minutes) to allow the starter motor to cool?
Is the ground power being used because it is very cold out? A proper engine preheat is a good idea in the case too then.
If you are the pilot starting on ground power at night, have a flashlight close at hand. When you signal the removal of the ground power plug, the electrical system will be de-energized, and all those nice cockpit lights will go out suddenly. You'll either be feeling for, or using your flashlight, to find the master switch...
The proposal to use ground power on a 172 "pops up a few little red flags" for me...
Is the ground power necessary because the battery was found to be discharged? Cause found and corrected before flying attempted again? (usually master was left on - but be sure it's not another failure). Could the battery have frozen while discharged? Frozen battery = buy a new battery before flying.
Is the ground power necessary because it was run down trying to start? It should have started, was the reason for not starting found and corrected? If so, has a suitable period of time elapsed (30 minutes) to allow the starter motor to cool?
Is the ground power being used because it is very cold out? A proper engine preheat is a good idea in the case too then.
If you are the pilot starting on ground power at night, have a flashlight close at hand. When you signal the removal of the ground power plug, the electrical system will be de-energized, and all those nice cockpit lights will go out suddenly. You'll either be feeling for, or using your flashlight, to find the master switch...
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If the Master switch is ON (ie both BATT and ALT together) when ground power is attached and the engine is cranked, I am led to believe this will fry your alternator over time. The more technically brilliant may be able to explain or deny this.
Hence I concur with the other posters; master switch off for ground power starts; disconnect ground power; master switch on. That's the Piper POH procedure.
Hence I concur with the other posters; master switch off for ground power starts; disconnect ground power; master switch on. That's the Piper POH procedure.
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Thank you so much for the replies. A&C and Pilot Dar, you both confirmed my memory. The POH with which I was provided was somewhat ambiguous on the point so where better to ask than here? The reason for the flat battery was elapse of time and the cold weather but in the event all went well and ground power was not needed so the pilot and his aircraft are now back in the air.
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Sitting here with a C182S manual in front of me.
Apart from switch off before plugging in, and don't switch avionics on until AC is started, there is no other detail. Seems non critical from then on.
DO.
Apart from switch off before plugging in, and don't switch avionics on until AC is started, there is no other detail. Seems non critical from then on.
DO.
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It's clear in the 172 POH (mine's a M 1974 model) :
"When using an external power source, the position of the master switch is important. External power may be used for cold weather starting and during lengthy maintenance work on the electrical system. Just before connecting an external power source, the master switch should be turned on. Battery and external power circuits have been designed to completely eliminatethe need to jumper across the battary contactor to close it for charging a completely dead battery. With a dead battery and external power source applied, turning on the master switch will close the battery contactor."
Same info in the C152 manual with additional note "Turning on the master switch is especially important since it will enable the battery to absorb transient voltages which otherwise might damage the transistors in the electrical equipment."
"When using an external power source, the position of the master switch is important. External power may be used for cold weather starting and during lengthy maintenance work on the electrical system. Just before connecting an external power source, the master switch should be turned on. Battery and external power circuits have been designed to completely eliminatethe need to jumper across the battary contactor to close it for charging a completely dead battery. With a dead battery and external power source applied, turning on the master switch will close the battery contactor."
Same info in the C152 manual with additional note "Turning on the master switch is especially important since it will enable the battery to absorb transient voltages which otherwise might damage the transistors in the electrical equipment."
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I checked the POH for a 1967 Cessna F172H and it says “Just before connecting an external power source (generator type or battery cart) the master switch should be turned “ON””
It also goes on to say that the external power cannot be used to power the onboard electronics only the aircraft electrics. The bus bar is split and the electronics bar is isolated during external power provision to protect any semiconductors in the electronics from damage by transients in the external power source.
It also goes on to say that the external power cannot be used to power the onboard electronics only the aircraft electrics. The bus bar is split and the electronics bar is isolated during external power provision to protect any semiconductors in the electronics from damage by transients in the external power source.
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Caution if external power is being used because of a flat battery. The high charge rate when the alternator comes on line doesn't do a lot for for battery longevity and has been known to cause battery boiling.
