Forum: F-16 Procedures

Hi all, pardon me that I have to ask a few questions on the falcon's hyd systems. I just need an answers to my questions so as to improve my sys knowledge.

1) Why is the pressure for flight control accumulator dump automatically upon shut down?

2) Why retain the hyd sys reservoir pressure when the above-mentioned is dump? is it just a pure case whereby its for the ground crews to monitor pressure?

Can the pros guide me on this cause I really could not find the ans after going through our technical manuals. Or if anyone can provide me with info like where can I find the real ans, it will be great.

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Enthusiast Joined: Sep 26, 2006 - 10:26 PM Posts: 55

29GS TO will give you all the answers

rupp05 wrote:

29GS TO will give you all the answers

thanks for the fast reply, but I did went through them. GS only mentioned the components description but did not talk about anything about holding pressure for what purpose. It also did not mentioned that the purpose of why the flight control accumulator will dump pressure upon shut down.

please correct me if i'm wrong.thanks

There is no checkvalve or other type of shut-off valve between the flight control accumulators and the system (A or B) that they are connected to so pressure is lost within a few seconds after shutdown. Be aware that the flight control can make sudden deflections during shut-down.
The reservoirs retain pressure after shut-down to make it easier for the hydraulic pumps to build up pressure during start-up.
Hope it helps.

jetfuelstarter wrote:

There is no checkvalve or other type of shut-off valve between the flight control accumulators and the system (A or B) that they are connected to so pressure is lost within a few seconds after shutdown. Be aware that the flight control can make sudden deflections during shut-down.
The reservoirs retain pressure after shut-down to make it easier for the hydraulic pumps to build up pressure during start-up.
Hope it helps.

I don't think that's what he was asking. Under operating pressure both the flight control accumulator and reservoir accumulator gauges read 3000 psi +/-, but when operating pressure is removed the flight controls automatically dump to the nitrogen pressure while you have to manually dump the reservoir pressure. I can't say for sure, but my best guess is that keeping pressure on the reservoirs allow you to bleed the system if it's been over-serviced or has air in it. Also (I believe) the flight control accumulators aren't actually building up or storing any hydraulic pressure, they're just receiving a steady flow of pressurized hydraulic fluid; when the pumps stop turning there's no longer any supply of pressure to the accumulators so it drops off.

All accumulators in the F-16 remain pressurized with hydr.pressure after shutdown. Except the flight contol accumulators . Thats because the FLC accumulators are designed to deliver pressure when the supply pressure decreases. They deliver pressure to stabs, flaperons and rudder only to compensate for press fluctuations in the system and EPU transfer.
I agree that a pressurized reservoir allows servicing but that is not the main purpose.

The primary purpose of the system A & B hyd accumulators is to maintain reservoir pressurization ensuring there is always positive flow to the pumps. As system pressure rises to 3000 psi, that pressure overpowers the 750 - 850 psi of nitrogen pre-charge pressure already in the accumulator. The pressure is held in the accumulator by the Reservoir Service Manifold Pressure Check Valve until needing to be released due to fluctuations in reservoir pressure (dropping below 3000psi). At engine shutdown the 3000 psi remains in the system. That valve is why the reservoir dump valve dumps the accumulator pressure as well. By depleting reservoir pressure you force the accumulator to release it pressure in attempt to maintain the reservoir pressure.

As far as the flcs accumulators go, I don't exactly know what causes them to dump automatically...

Clear as mud?? Evil or Very Mad

[/quote]
As far as the flcs accumulators go, I don't exactly know what causes them to dump automatically...
[/quote]

As jetfuelstarter already mentioned, the FLCS accumulators are the only accumulators that doesn't keep there pressure comparing to the rest of the reservoirs. It shows how important the primary flight controls actually are for the F-16.
There is a flight control pressure checkvalve in each hydr. system. They ensure that the energy in the accumulator is ONLY used for the PRIMARY flight control ISA's (rudder, horizontal stabilizer and flaperons). It is a DIRECT connection from the accumulator to the ISA's.
Especially during a dual FLC fail, the hydraulic energy in the accumulators will give a moment of control, just enough time to allow the emergency hydraulic pump to deliver hydraulic pressure to pressure system A (to continue control of the primary flight controls and all other systems connected to system A (LEF, speedbrakes, FFP))
When both hydraulic systems go down, by fail or a normal engine shutdown, all 5 ISA's go to the center mode because there is no pressure anymore and there is a centering spring in the ISA that forces the main control valve and main power ram of the ISA to go to the neutral (streamlined) position, so THAT is why the flight controls as fuelstarter mentioned move a bit erratic after engine shutdown and off course causes a movement in the hydraulic system, and thereby loosing hydraulic energy in the accumulator and displaying precharge.

Hope you know enough now Wink

You can check the connections and positions of the FLC checkvalve and ISA's in the schematic diagram, 12-29-11-02 for system A and 12-29-12-02 for system B in the 29FI.

jetfuelstarter wrote:

All accumulators in the F-16 remain pressurized with hydr.pressure after shutdown. Except the flight contol accumulators . Thats because the FLC accumulators are designed to deliver pressure when the supply pressure decreases. They deliver pressure to stabs, flaperons and rudder only to compensate for press fluctuations in the system and EPU transfer.
I agree that a pressurized reservoir allows servicing but that is not the main purpose.

Just to be clear, and not to start an argument, I wasn't trying to say that the reservoirs are only pressurized to allow servicing, just that a reason for them to stay pressurized after operating pressure is removed is to allow for servicing.

Bodizzle wrote:

Just to be clear, and not to start an argument, I wasn't trying to say that the reservoirs are only pressurized to allow servicing, just that a reason for them to stay pressurized after operating pressure is removed is to allow for servicing.

Some more info on the reservoir accumulators:
If they would not stay pressurized you also would never be able to take an hydraulic oil sample from the fluid sampling valve in the return filter manifold for a contamination check.
And if they stay pressurized the engine driven hydraulic pumps need less effort for starting up because the big suction tube is still pressurized (jetfuelstarter also mentioned that).
To check if the reservoir has the correct percentage of hydraulic fluid you need be sure that it is or fully pressurized or completely dumped to precharge (depressurized) to make sure you use the correct procedure for checking the right amount of hydraulic fluid (described on table on inside of servicing panel/door).
The status of the nitrogen pressure can only be checked if you manually dump the hydraulic press back into the reservoir.
So it depends on what kind of "servicing" you wanna do when the reservoir is pressurized Wink

Thanks for all the numerous replies, it does enhances my knowledge, however I think I need to be clearer abt my questions. I know that the reservoir needs to be pressurized so that samples can be collected but the thing is, after shutdown the ground crew will dump it after verifying sys pressure of 3k coupled with correct volume at the temp sensed. So why must they dumped it themselves? as in why is the falcon designed to let them do this check and then dumping pressure after verification?

FLC accumulators does not have any check valves to hold pressure thus it will lose its pressure upon shutdown, it can be seen in the schematic diagrams. However, Why is it designed not to be manually dump? As you all know, even when the sys is pressurized the DCC also will be walking around the aircraft for checks so I do not think that the explaination for the FLC to have slight movement during shutdown is really the concern for it to automatically dump.

Sorry if i'm wrong but over at my side, the reservoir will be dumped of hyd pressure after shutdown. We do not keep the reservoir pressurized so that next start will be easier.

simonlwa wrote:

So why must they dumped it themselves? as in why is the falcon designed to let them do this check and then dumping pressure after verification?

FLC accumulators does not have any check valves to hold pressure thus it will lose its pressure upon shutdown, it can be seen in the schematic diagrams. However, Why is it designed not to be manually dump?

You should really read again what is explained by me and fuelstarter again because that was already explained by us. (just to give some idea we both have around 20 years of experience on the F-16 and also have been teaching this stuff, so i can go further into detail but the explanation given was already very thorough. The tricky thing about knowing the hydraulic system is knowing ALL the individual hydraulic systems by itself and what influence they have upon each other in different kind of situations, so we keep learning every day)

F-16 hydraulics according to Varkvet.

If you look at the 6 WC there is nothing that tells you to dump your reservoir accumulators upon engine shutdown or while performing any inspection. Dumping is just a standard crew dawg procedure to relieve system pressure for two reasons. 1. You relieve system pressure so your jet doesn’t sit there and start leaking all over the ramp like F-4’s and Wonder lemons. 2. Dumping the system is a good way of seeing how much air is in your jet … Check fluid level at 3000 psi and then again at pre-charge, when you dump it, B system fluid level should not increase more that 3% and A no more than 5%.

Thanks for all your replies guys...I'm much clearer about it. So sorry about keep asking but think I mis-interpret some of the replies thus resulting in me seeking clarifications. Cheers...Salute to your knowledge. No offense, I'm just trying to learn more about the aircraft I maintain.

In the 6 WC it does say to inspect you precharge levels. In order to do that you have to relieve the pressure.

Forum: F-16 Procedures

Why retain hyd reservoir pressure?