737NG flight Altitude Indicator

[Pin Head]

Hi

Saw a new one yesterday. In a slow gradual step down descent any benefit in resetting the new lower cruise altitude in the flight altitude window?

[Chesty Morgan]

You mean the flight altitude window on the MCP?

[Pin Head]

Hi. No that's the Altitude display.

It's on the pressurisation panel.

[RVF750]

None at all. it will step down until max differential and hold the cabin until your next step so as long as it's set for landing altitude correctly it's set and forget.

Unless you that bored.....

Won't hurt but remember the FL370 reschedule so you might get funny stuff if you go from above that to below.

[framer]

Boeing has released a Bulletin or similar recently advising against doing the reverse of this in the climb. I can't remember exactly where I read it but I think it was a Bulletin. The aircraft will schedule it nicely even with step climbs/ descents and the more you play with it the more likely you are to c@ck it up was the guts of it.

[None]

This?

Investigation of pressurization incidents in the fleet has indicated that some
operators are setting the FLT ALT before takeoff to an intermediate altitude and
then resetting the FLT ALT one or more times during climb until the final cruise
altitude or flight level is reached. Although this method of operation also provides
a safe and comfortable cabin altitude for the duration of the flight, there are some
considerations which may make this method of operation less desirable:

• Multiple adjustments of the FLT ALT unnecessarily increase crew
workload during climb.

• Intermediate FLT ALT settings are likely to result in higher cabin altitudes
than if the final planned cruise flight altitude is selected. This is because
the pressure schedule for lower altitudes uses lower differential pressure
limits.

• If the flight crew does not reset the FLT ALT when appropriate, flying
above the selected FLT ALT can result in an overpressure situation and
activation of the pressure relief valves.

[RAT 5]

Seems like some guys just like to try and reinvent the wheel and think they can do somethings better than the automatics. True, sometimes they can, but it's knowing when & where. In older a/c it was much more possible to manipulate the pressurisation. On one occasion I had a lady passenger with possible internal bleeding problems, but the overseas Doc said it was OK to take just the one flight to repatriate. It was only 3 hours. We could file a lower level and we could manipulate the pressurisation to fly high but keep the cabin almost at sea level; just a possible preventative precaution. I once tried the same thing with a more modern a/c and made a complete horlicks of it because it was 'set it and leave it'. Other than complete manual override it was not possible to intervene.
Sounds like one operator who used V/S to clean up the flaps rather than TOGA & Bug Up. That was until they for got and made a horlicks of the next phase.
KISS is an SOP!

[Pin Head]

Great stuff guys.

Ps do you have the bulletin number.

Flying in a part of the world where they are trying to invent the wheel. Not good.

[slr737]

None,

Was it a bulletin ? Or something else ?

could you forward it to me in original version ?

[Mikehotel152]

My airline has a SOP of always setting the flight altitude panel to match the cruise level in the FMC. As we're a shorthaul operator this doesn't usually cause a problem, but where the max cruise level in the FMC is below the intended cruise and a step climb is needed, there is a need to re-set both the FMC and the pressurisation panel once cleared to the higher level. This regularly leads to a change of differential and a cabin descent as the step climb commences.

I know it is possible to set the intended final cruise in the pressurusation panel nothwithstanding the FMC max cruise restriction, and I inadvertently did just that when just out of line training, to the severe bollocking of my Captain. Was he guarding against some danger or merely pedantically following SOP? Alternatively, would setting the final cruise level in the pressurisation panel lead to a more linear pressurisation regime?

[Skyjob]

Setting the highest expected cruise level SHOULD be done and not your company's existing SOP for which you got incorrectly bollocked by a captain following blindly his taught SOP.

The reference quoted is in FCTM not a bulletin

[Mikehotel152]

Just to be sure we're not talking about different scenarios Skyjob, are you saying that the highest pressurisation cruise altitude should be set even where it is higher than a (weight-restricted) cruise altitude set in the FMC?

[Chesty Morgan]

Originally Posted by Mr Boeing

The Cabin Pressure Control System allows the flight crew to set the planned cruise flight altitude (FLT ALT) and the altitude of the intended landing field (LAND ALT). These altitudes are then used by the pressurization system to calculate a pressure schedule specific to the planned flight. The pressurization system is designed to maintain a comfortable and safe cabin altitude throughout the duration of the flight based on the FLT ALT and LAND ALT set during preflight. The only time a change in FLT ALT or LAND ALT is required in flight is when the final cruise or landing altitude is different than that selected during preflight

Yes!......

[Pin Head]

Chesty can I ask where that txt comes from. FCTM, Systems?

[Chesty Morgan]

Yes, FCTM. Chapter 2 - Ground Operations - Pre Flight.

[737aviator]

Surely if one sets the 'highest expected cruise level' on a 5 hour flight, and never makes it that last 1/2000ft due to a variety of reasons, and commences their descent they'll then be triggering an 'Off scheduled descent'? Its our SOP to set our highest expected initial cruising altitude, and several hours later if we wish a step climb, to reset the pressurisation panel when actually cleared to climb.

[Chesty Morgan]

No, as per the FCTM if you don't make your planned cruising level then you reset it to the achieved flight level.

[RAT 5]

Its our SOP to set our highest expected initial cruising altitude,

That would suggest that is what you do before takeoff. What you do when at final cruise FL is, surely, is set what you are actually doing, not what you expected to do?? Why would you leave an error set and wait for a warning system to tell you that an error had been made?

[Skyjob]

Some airlines are selecting initial CRZ LVL pre-flight, and although this is not unsafe it is not the recommended method from Boeing.

Yes, Mikehotel152, it is my understanding that if a higher expected CRZ LVL is to be used later in flight then this should be set pre-flight, as per FCTM.

A large EU LoCo has the procedure to set initial CRZ LVL, many others do not...

The reason for Boeing to recommend the highest expected CRZ LVL to be set is to select the higher differential pressure PRE-flight rather then create the pressure change in-flight when selecting a CRZ LVL above FL370. The latter would cause cabin altitude to decrease, at a rate of approximately 1000'/min rather then the default descend rate of ~300'/min in normal descend, all while the aircraft is providing full CLM thrust attempting to reach the new selected CRZ LVL, until the cabin controller has achieved the cabin altitude it should be at during the climb and follow the pressure differential now (newly) selected until TOC.

[de facto]

Boeing philosophy: If it is working ,dont f touch it.
Always set the highest achievable FL in climb,thereby avoiding multiple resets,reduces crew load,avoids overshoots and all the ****s that follows.
To avoid off schedule descents ,which aint a problem really,then a standard procedure of a descent checklist before descent should be standard.....