FLT / GRD switch in Pressurization Panel.

[Bulva]

I have such a stupid question.
I read and more or less understand the operation of the FLT / GRD switch in different modes (AUTO, STBY, and MAN).
But I wonder, in what real situations does the switch switch to the FLT position ???

 

And, as a side note, cool site :

https://flight-school.do.am/

Edited July 25, 2021 by Bulva

[Hotdawg]

You place the FLT/GND switch to FLT during the after start flow and return it to GND during the after landing flow. In the FLT position the cabin is pressurized to about .2 psi or 200 feet below field elevation. If you forget an leave the switch in GND the cabin will still pressurize normally but you will get a pressure bump when the weight on wheels switch allows the outflow valve to close after takeoff.  

[Bulva]

Thanks.

This information confused me a bit:

I understood it as if there was no need to manually switch to the FLT position after takeoff, and the system would automatically behave like in the FLT position.
I watched a lot of movies on YT and I haven't seen anyone manually switch from GRD to FLT after takeoff  - hence my question and doubts. ;-)

 

Now, I can see the detailed checklists say this:
GRD -> FLT - po START-UP engines
FLT -> GRD - AFTER LANDING

Edited July 26, 2021 by Bulva

[Litjan]

The reason and explanation is in the post above yours .

The reason for the "pressure bump" Hotdawg mentioned is that the outflow valve would be in the "far open" position, leaving a big hole in the aft lower section of the fuselage. As the plane rotates, air pushes against and into that hole, creating (uncomfortable) overpressure in the cabin.

If you place the switch to FLT while still on the ground, this hole is almost closed, as the outflow valve is almost closed.

Another benefit is that the cockpit sliding windows do not "rattle" in the guiding rails, they are pressed firmly into their sockets by the overpressure.

Cheers, Jan

 

[Bulva]

5 hours ago, Litjan said:

Another benefit is that the cockpit sliding windows do not "rattle" in the guiding rails, they are pressed firmly into their sockets by the overpressure.

Is window "rattling" (or not) simulated in IXEG B733 ?  

[Litjan]

2 hours ago, Bulva said:

Is window "rattling" (or not) simulated in IXEG B733 ?  

No - but try to open the sliding window after putting the FLT/GRD switch to FLT(with packs operating).

Edit: To FLT, obviously.

Edited July 26, 2021 by Litjan

[Bulva]

Sorry, but I have a small request to clarify the procedure for switching from FLT to GRD after landing.
When exactly should this be done? Before or after turning off the engines?
According to the checklist:
AIR COND & PRESS
(AS INSTALLED) .............................................. ............ 1 PACK, BLEED ON, GRD

Is it normal that the CABIN CLIMB indicator jumps momentarily to very high values when switching from FLT to GRD ???

 

[Litjan]

The FLT/GRD switch is moved to GRD with the "after landing items" - as you leave the runway.

The outflow valve will drive to "FULL OPEN" very fast, and this "dumps out" the small positive pressure in the cabin, resulting in a momentarily high cabin rate of climb (remember, more pressure -> cabin "descends", less pressure -> cabin "climbs").

Try to open the outflow valve when flying at 30.000 feet and see what happens with the cabin rate .

PS: Are you aware of the folder "documentation" contained in the aircraft folder? There is a .pdf in there called: 3-Pilot Quick Reference Handbook. It contains the normal procedures for operation, and may answer a lot of your questions (as it also contains some annotation for WHY things are done).

Here is the excerpt for Chapter 6:

CHAPTER 6
AFTER LANDING AND PARKING
Taxiing the aircraft to the parking position or gate after landing can be very demanding, too.
It takes good crew coordination to keep situational awareness, in addition to performing
some steps to prepare the aircraft for arrival at the gate.
AFTER-LANDING ITEMS:
Reversers stow
Speedbrake down
Landing lights as required
• Usually turn off the landing lights and turn on taxi-lights and runway-turnoff lights
Flaps up
Pitot-Static-Heat off
• This removes heating from the pitots during ground operation, they would get too
hot without the cooling airflow.
FLT/GRD Switch GRD
• This will open the outflow valve, depressurizing the airplane so you can open the
cabin doors later on.
Strobe Lights OFF
• They could blind other aircraft/personnel
Engine start switches OFF
APU As required
• Only use the APU if you need it for electrical power or aircondition after parking. Its
loud and uses fuel!
Single-engine taxi consider

Edited July 27, 2021 by Litjan

[Bulva]

5 hours ago, Litjan said:

PS: Are you aware of the folder "documentation" contained in the aircraft folder? 

Yes, of course I know it. The documentation is great.
Thank you very much for your help once again.
It is a real pleasure to get to know and explore this plane, when it is so realistically reproduced.
Thanks ;-)

[Bulva]

I can see that the object of my interest even has smooth animation ;-)

[Litjan]

Yes, and the blurry spot above it is the overpressure relief valve that opens to keep maximum differential pressure at 9.something psi, iirc.

[Beachdog2001]

This subject is an Interesting nuance I never noticed before. Thanks for bringing that up.