737 NG duct pressure differential limit

[RMC]

I have in my hazy memory 15 PSID in flight and 10 PSID on the ground. Cannot remember where I got this from... and hence whether it is actually correct?

Thanks in advance.

[IFixPlanes]

Quick and dirty acc. TSM:
Regulated pressure when engine is above 50% N1 should be 42 ± 8 psi. So you can get a split of 16 psi.
Regulated pressure when engine is below 50% N1 should be 32 ± 6 psi. So you can get a split of 12 psi.

[BOAC]

IFix - do I assume these were Eng 'troubleshooting' limits as I do not ever recall having them as 'operating limits', and were they NG only?

[FCS Explorer]

Duct Pressure Transmitters

Duct pressure transmitters provide bleed air pressure indications to the respective (L and R) pointers on the bleed air duct pressure indicator. The indicator is AC operated. Differences between L and R duct pressure on the bleed air duct pressure indicator are considered normal as long as there is sufficient air for cabin pressurization.

[IFixPlanes]

I wrote "acc. TSM". Thats the Trouble Shooting Manual.
The values for the CL are the same.

Boeing said in the service letter 36-013-B that if inflight duct pressure is below the values mentioned above but more than 18 psig,
...the airplane can be operated normally (without imposing the MEL), but action should be taken to investigate and restore the bleed system performance to normal operation at a convenient opportunity when manpower and equipment is available.@ FCS Explorer
Only the Indicator of the CL is AC operated (28V AC).
The Indicator of the NG is only supplied by 115V AC. But it is operated by 7V DC.

[MrHorgy]

Does anyone have a copy of that service bulletin, or the page from the TSM? Many people I fly with see a difference and fill their pants, it'd be nice to have something to wave at them.

Horgy

[Yeelep]

Can't help you with copies, your management should be willing to provide them for you as it would prevent unnecessary write ups. You could also ask someone in maintenance. The references that could prove useful are 737MT 36-006, 737-SL-36-013-B and AMM 36-11-00-710-801 Fig. 502. While the Service letters and maintenance tips are useful, they can't be used to sign off write ups. The AMM or FIM are the references that I would have to use.

Per 737MT 36-006. Normal ops are:
Ground/taxi: WTAI off 18-22, WTAI on 12-14
Takeoff: 37-50
Climb: 37-50
Cruise: 26-45
Idle descent: WTAI off 18-25

The book FCS Explorer is referring to is the Flight Crew Operations Manual, Section 2.20.2 Duct Pressure Transmitters: "Differences between L and R duct pressure indicators are considered normal as long as there is sufficient air for cabin pressurization". Is this all the guidance/info provided to you guys/gals at the pointy end of the plane? :sad:

The problem I have with the numbers IFix gave in his first post are that they aren't allowable split numbers, just the tolerance. You could have 35PSI on one side and 33PSI on the other with say 70% N1 and be out of limits.

Per an addition my airline made to our AMM (shortly after the SL came out), we are now allowed a 20PSI split as long as the pressures are between 18PSI and 60PSI. Over 20PSI a 10 day deferral to troubleshoot is applied. Below 18PSI or above 60PSI we MEL or repair the plane.

[RMC]

Thanks guys

[BOAC]

Is this all the guidance/info provided to you guys/gals at the pointy end of the plane - yes, that is all I ever had.