Fly Lah!

Not only because the LP cock is also used is case of malfunction of the HP cock or for isolating the HP circuit for maintenance intervention. In case of shutdown by the LP cock, ground staff must be cautious as it takes an abnormal time to stop.

True indeed Terengganu01...

In case of HP cock malfunction, LP cock will do the job as a safety device to overcome the HP cock malfunction thus supplying the fuel into the engine.
So, if the HP cock is under cavitation or vapour locking, is it true LP cock will take in action to overcome this problem?

The pressure in the HP part is quite high: 300/400 PSI at iddle are frequent. I do not see cavitation or vapour locking in the HP part of the circuit, on contrary it should occur in the LP part (tanks, LP pumps...). It is an opinion because I have not a certainty on this last point. To check....

-- Sat Jun 27, 2009 8:28 am --

I confirm vapour lock may occur in the tanks, at the level of the booster pumps (LP). It is mainly an issue when using some high volatibility (military) fuels (JP4....). The solution is to pressurise the tanks, particularly for supersonic aircraft (fighters). There are also some patented by-pass systems, by-passing the LP stage of a multi stage pumps (one LP stage and one HP stage generally): all this confirms the vapour lock is at low pressure, increasing the pressure is the solution to prevent it, it is a phenomenom occuring in the LP part.
Regards
Terengganu01

Terengganu01 wrote:The pressure in the HP part is quite high: 300/400 PSI at iddle are frequent. I do not see cavitation or vapour locking in the HP part of the circuit, on contrary it should occur in the LP part (tanks, LP pumps...). It is an opinion because I have not a certainty on this last point. To check....

-- Sat Jun 27, 2009 8:28 am --

I confirm vapour lock may occur in the tanks, at the level of the booster pumps (LP). It is mainly an issue when using some high volatibility (military) fuels (JP4....). The solution is to pressurise the tanks, particularly for supersonic aircraft (fighters). There are also some patented by-pass systems, by-passing the LP stage of a multi stage pumps (one LP stage and one HP stage generally): all this confirms the vapour lock is at low pressure, increasing the pressure is the solution to prevent it, it is a phenomenom occuring in the LP part.
Regards
Terengganu01

It's true..i just know cavitation and vapour lock can occur in LP system in the FCU..actually, most of cavitation problem and vapour lock is occured regarding the fuel pump in the LP system itself..but it's rarely happen on HP system in FCU since it use high Px in the system..anyway,thanks for your answer..

It depends on aircraft type, but generally:

- For turbine engines, moving the engine start lever to cutoff closes both the LP & also the HP valves. The LP valve is normally located somewhere on the wing, not on the engine itself while the HP valve is located on the engine itself, normally within the fuel pump. But this may differ slightly between aircraft types.

- As for piston engines, the proper way to shutoff the engine is by cutting off fuel (by pulling the mixture lever all the way out to 'cutoff'). Although turning the ignition key to "off" can also shut down the engine, but this is considered bad practice as there will still be fuel in the lines to the engine.

rollingcouple wrote:True indeed Terengganu01...

In case of HP cock malfunction, LP cock will do the job as a safety device to overcome the HP cock malfunction thus supplying the fuel into the engine.
So, if the HP cock is under cavitation or vapour locking, is it true LP cock will take in action to overcome this problem?

hi friends i have a question here, what push button inside the cockpit energizes the lp cock ? is it the boos-pump pushbutton itself?

Monkey Wrench wrote:

hi friends i have a question here, what push button inside the cockpit energizes the lp cock ? is it the boos-pump pushbutton itself?

Hi there..

Regarding your question, i can assume that you want to know how to energizes the LP Cock in the fuel system.

In FCU system, we can energizes the LP cock by the toggle switch in the cockpit area..FYI, it's normally operated by AC..normally it is design in form of toggle switch..you can look it in fuel control panel in the cockpit..it is normally situated in between the fuel inter stage switch and low pressure switch warning light.

so what kind of power source is used for lp corck ac or dc power? on the airbus a320 i haven't seen any separate switch for the lp cock so i assume that it is inter connected to the boostpump assembly where when the boost pump switch is turned on lp cock valve opens and fuel from tank is feed to the hmu itself . when hp cock from the master switch on the throttle console is switched to on the hp cock from the hmu outlet fuel will then be feed to the fuel nozzles .

Monkey Wrench,
Are you working on the A320?
Tell me, with regards to the fuel system, when you push the engine FIRE switch, what happened?

Very nice question and answer thanks friend.