Photo: Left Hydraulic System Manifold, from Eddie's aircraft.

Eddie Sez:

An entire page on hydraulic system DPIs? Why is that necessary? It is because there is some confusion about them and when they can be reset by a pilot and when they cannot. It helps to know where they are and which one of those little buttons belongs to which system. And then all you really need to know is you, the pilot, can only reset one of then ten. For the other nine, it takes a mechanic and a check procedure.

They teach you in school that the left system has five DPIs, the right system has three, and the Aux and PTU each have one. That is wrong. Think of it this way. Each DPI is a check on a filter and there are ten in total:

  1. The left engine pump has a bypass filter, located on the left manifold.

  2. The right engine pump has a bypass filter, located on the right manifold.

  3. The left hydraulic system has a main pressure filter, located on the left manifold.

  4. The left hydraulic system has a main return filter, located on the left manifold.

  5. The right hydraulic system has a main pressure filter, located on the right manifold.

  6. The right hydraulic system has a main return filter, located on the right manifold.

  7. The PTU has a bypass filter, located on the left manifold.

  8. The PTU has a pressure filter, located above its dedicated filter above the PTU itself.

  9. The Aux pump has a return filter, located on the left manifold.

  10. The Aux pump has a pressure filter, located above its dedicated filter below the aux pump itself.

What follows is taken from the references shown below and where I think necessary, my comments shown in blue.


The Left Hydraulic System Manifold

Figure: Manifold Filters and Differential Pressure Indicators, from G450 Maintenance Manual, §29-14-01, Figure 601, Detail B.

Much of what appears in the G450 Maintenance Manual comes from the GIV Maintenance Manual and in that airplane the left system is called the "combined system." Looking at the manifold, you see five DPIs of which only two belong specifically to the left system, one belongs to the engine, one belongs to the "Utility Pump Bypass Filter" (which was what the PTU was called in the GIV), and the final one belongs to the auxiliary pump.

You, as a pilot, are not allowed to reset any of these. It takes a mechanic who will then have to run the procedures in the G450 Maintenance Manual, §29-14-01, Page 603. (This requires the system be run at "full pressure and maximum flow during normal operation" followed by a check of the DPI. If it pops again, the filter must be replaced.)


The Right Hydraulic System Manifold

Figure: Manifold Filters and Differential Pressure Indicators, from G450 Maintenance Manual, §29-14-01, Figure 601, Detail A.

Much of what appears in the G450 Maintenance Manual comes from the GIV Maintenance Manual and in that airplane the right system is called the "flight system." Looking at the manifold, you see three DPIs of which only two belong specifically to the right system, one belongs to the engine.

You, as a pilot, are not allowed to reset any of these. It takes a mechanic who will then have to run the procedures in the G450 Maintenance Manual, §29-14-02, Page 603. (This requires the system be run at "full pressure and maximum flow during normal operation" followed by a check of the DPI. If it pops again, the filter must be replaced.)


The Aux Hydraulic System Manifold

Figure: Auxiliary pump pressure filter DPI, from Eddie's aircraft.

The aux pump pressure filter DPI is on the left side of the aircraft, underneath the aux pump itself.

You, as a pilot, are not allowed to reset this DPI. It takes a mechanic who will then have to run the procedures in the G450 Maintenance Manual, §29-25-01, Page 601. (This requires the system be run at "full pressure and maximum flow during normal operation" followed by a check of the DPI. If it pops again, the filter must be replaced.)


The PTU Manifold

Figure: PTU filter DPI, from Eddie's aircraft.

This is the only DPI of the ten you can reset. Why are you allowed to reset it? Because the AOM says you can:

[G450 Airplane Flight Manual §2-01-20]

  1. Power Transfer Unit (PTU) DPI (1) — Check
  2. NOTE: Ensure DPI is flush. If DPI is popped, reset DPI and note time in logbook. Continued operation for the next fifty (50) flight hours is allowed. Check DPI prior to each flight.


References

Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.

Gulfstream G450 Maintenance Manual, Revision 18, Dec 12, 2013