Boeing 737 Memory items – Cabin Altitude Warning / Emergency Descent

JohanAero

When something goes wrong while flying on the Boeing 737, we use the QRH (Quick Reference Handbook). We just have to find out the correct NNC (Non Normal Checklist) and then we just read it and execute it.

For some of these checklists, we have memory items, meaning we have to know by heart some actions.

I have listed 9 memory items:

Cabin Altitude Warning / Emergency Descent

Don oxygen masks and set regulators to 100%.
Establish crew communications.
Pressurization mode selector ......................................MAN
Outflow VALVE switch ....................................Hold in CLOSE
                                               until the outflow VALVE
                                         indication shows fully closed
If cabin altitude is uncontrollable:
     Passenger signs ...............................................ON
     PASS OXYGEN switch ............................................ON
     >> Go to the Emergency Descent () checklist
Announce the emergency descent.The pilot flying will advise the cabin crew, on the PA system, of impending rapid descent. The pilot monitoring will advise ATC and obtain the area altimeter setting. 

Passenger signs ....................................................ON 

Without delay, descend to the lowest safe altitude or 10,000 feet, whichever is higher. 

ENGINE START switches (both) .....................................CONT 

Thrust levers (both)..................................Reduce thrust to
                                                         minimum or as
                                                   needed for anti-ice                                                                        

Speedbrake...............................................FLIGHT DETENT

If structural integrity is in doubt, limit speed as much as possible and avoid high maneuvering loads.

Set target speed to Mmo/Vmo.

Both the Cabin Altitude Warning and the Emergency Descent checklists are likely to be used together.

So basically, when there is a problem with the pressurization system, we will get the CABIN ALTITUDE WARNING and we will hear a strong and annoying horn in the cockpit as you can see in the following video.

When the problem is identified, the first thing to do by the crew is to put on the oxygen masks and establish communication. When this is done, it is possible to cut off the horn, what they obviously don’t do in the video. Then you can work properly and in a less anxious atmosphere. 

Now the First Officer will put the pressurization mode selector to manual and close the outflow valve, trying to control the cabin altitude. If it works, then fine. If it doesn’t, then First Officer and Captain will do memory items for the emergency descent.

This can be slightly different from one company to another.

First Officer role:

Passenger oxygen: On
Seat belts: On
Landing/Turnoff lights: On
Start switches: Continuous
Transponder: 7700 and TA only. (Traffic Advisory only)
Declare Mayday and request QNH
Verify Captain actions and monitor.
Call TWO TO GO, ONE TO GO when approaching level off.

Captain role:

Over the Public Announcement system: EMERGENCY DESCENT X3.
On the MCP Altitude panel: Set 10000ft or lowest safe altitude.
Select LEVEL CHANGE.
Set a heading 45deg off the current heading and PRESS HEADING SELECT.
Manually close the thrust levers.
Put the speedbrakes to Flight Detent position.
Increase speed to VMO/MMO.
Reduce Rate of descent when approaching level off and retract speedbrakes.

Then the Captain will call for the NNC Emergency Descent, First Officer will go through and both verify they didn’t forget anything.

Finally, take some time to think about what to do next, diversion, inform number 1, pax, ATC…

It is important to remember that the most important thing is to put the oxygen mask as soon as we get the cabin altitude warning horn, especially in case of explosive decompression. This can lead to crew incapacitation due to hypoxia. Below is a table of the time of useful consciousness for different altitudes if you don’t put the oxygen mask.

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