Pressurization

Problem On a Boeing 737-300,400
Climbing to 27000 ft the pressurization system works fine7.45 psi, on the cross over to 28000 feet the system falls behind and seems to climb as the aircraft climbs pressure is above 7.8 and higher.

We have changed the controller panel, pressure controller and the Delta P no difference


Please advise

When you mention that the aircraft cabin pressure climbs with greater altitudes, my first guess would be that you've got a leak. Most likely a big one.

The packs can only output so much. Beyond a certain altitude..... they just can't keep up.

If ground time allows, I'd put somebody inside and take it to 3 or 4 PSI. A couple of guys can check the outside for holes, blown ducts, or bad door seals. If you have a leak, I bet you can probably hear it.

If the outflow valve is manually closed on the ground, can you verify that it is fully closed visually?

Did the crew noticed if the outflow valve was fully closed in flight?

I had a similar issue a while back. The packs could keep up, but the system warned of high pack inflow.

If time doesn't allow..... I'd ask the crew if they could try to bring the cabin down manually on the next flight..... If they can, the problem lies elsewhere.

After looking at the wiring and maintenance manuals, I really couldn't find an explanation of the function of the Delta-P unit.

One way to know for sure that a leak might be present is the rapid drop in differential when the packs are turned off during a ground check. The poor guy inside might have some pissed off ears if that is the case.

Also is that cabin Altitude climbs as we go above 28000 ft but the differential pressure stays at 7.45 psi it should go up to 7.8 psi

Now you're throwing an extra twist in here. You've only got two basic indications in flight.... cabin altitude and cabin differential. If cabin alt indication climbs, but the differential remains steady..... one of the two is wrong. Can the higher altitude be felt in your ears?

What is the crew complaining about? Indication or an actual high cabin altitude?