B737-800/900ER ATA 36 (PNEUMATIC) "Engine Loss And Possible Effect On Pneumatic System"

ATA 36 (PNEUMATIC)

Engine Loss And Possible Effect On Pneumatic System

The QRH provides information towards the safe handling of shutting down any engine but what effect has that on the bleed users and performance of the other engine, or what if we select an ENGINE BLEED AIR VALVE to close? A little further knowledge of the bleed air system will result in more understanding on the effect of losing an air source related to switch positions.

It all comes back at which engine has been shut down, or bleed system that has been closed in combination with the switch positions on the bleed panel, so let’s have a look at that. We identify in this situation a left, and right side of the bleed manifold separated by the ISOLATION VALVE which has a paramount role in closing or losing a bleed source.

On the left side these bleed air sources are engine #1 and the APU, where the right side sources are engine #2 and the external air connection. As external air has normally no effect on an operational bleed selection or malfunction I’ll not consider it in this post.

The systems that use bleed-air from the manifolds are mentioned next and indicate that most users take air from the left side of the bleed manifold.

Left side manifold:
– Engine #1 starting
– Engine #1 TAI
– Wing TAI left
– Air condition pack Left
– Hydraulic reservoir A system
– Water reservoir
– Nitrogen Generation System
– TAT aspiration (if installed)
– Auxiliary fuel tanks (if installed)

Right side manifold:
– Engine #2 starting
– Engine #2 TAI
– Wing TAI right
– Air condition pack right
– Hydraulic reservoir B system (plus standby)

A restriction by FCOM is to limit one source for the bleed air users such as the PACKS, as the FCOM states “do not operate two packs from one engine” which is of course related to engine performance and air demand. When non-normal procedures are followed you’ll see that the affected PACK is always selected to OFF to assure the previous. The use of WTAI when operating on one engine also affects the performance of the operating engine although the QRH guides you of course to the use of wing de-ice when needed. The use of WTAI requires more performance by air demand from the operating engine than for instance a PACK, where the PACK is selected OFF. In sequence the QRH commands to select the affected PACK to OFF and later verification of the ISOLATION VALVE at AUTO to make sure WTAI is available on both sides when needed. Remember that with the ISOLATION VALVE switch in AUTO and any “corner switch” OFF will open the ISOLATION VALVE.

The remainder of bleed air users (mainly on the left side of the manifold) do not affect the performance of the engine that much as they have a relatively small demand of air. But be aware that securing the left bleed supply (engine) and leaving the ISOLATION VALVE closed you’ll lose these bleed users and expect indications accordingly.

A loss of (left) bleed pressure affects:
– Hydraulic tank pressurization which can result in foaming and related blinking LOW PRESSURE light.
– WTAI (if ISOLATION VALVE is closed)
– The water tank is now pressurized by a system compressor.
– The NGS system is inoperative.
– The aspirated TAT indicator is not receiving air so it will affect correct indications.
– The Aux fuel tank pushes fuel to the center tank by cabin pressure and has no backup from the bleed system.

Note: TAI remains available when the engine operates with the ENGINE BLEED VALVE closed as it branches from before this valve directly from the engine bleed. (5th or 9th stage HPC)

Note: although available below 17.000 feet, (or 10.000 together with APU GEN operation) use of APU bleed air is not considered. The FCOM states that the APU is capable of supplying 1 PACK in flight and both PACKS on the ground.