TO Config warning in flight?

[martinlest]

Hi. Why would I get this alert (traced to toconfigwarn.wav) when I am at levelling off at cruise altitude? There were no annunciator alerts/warnings of any kind - everything looked perfectly normal (e.g. no inadvertent flaps), but the horn went on and on..  I found no way to shut it off and in the end had to switch off XP11 altogether to stop it. Frustrating after having been setting up and flying for almost an hour. 

Any suggestions as to the cause please? A bug??

Thanks!

Edited May 12, 2023 by martinlest

[Litjan]

This will probably answer your question - as the pilots asked themselves the very same question that you just asked

https://en.wikipedia.org/wiki/Helios_Airways_Flight_522

 

[martinlest]

Hi. Yes, I know of that incident, . I always move the pressurization switch to 'FLT' after takeoff and I always set the FLT ALT & LAND ALT values during flight preparation (part of my checklist).

The warning started the second I changed the cruise Mach value from 0.74 to 0.75 in the CRZ page of the FMC. I thought at first it was an overspeed warning, then  realised in was the TO config. I went over and over the switches, but as I say, everything looked just perfectly normal. In the end, no way I could fly on with that loud horn sounding. Had to cut the flight, as I said.

[Litjan]

Well, what was your cabin altitude? Just moving the switch to FLT and setting the correct FLT ALT and LAND ALT is not enough, as the Helios pilots found out, too.

The next time it happens please take a screenshot of your cabin altitude indicator and also of your pressurization panel (bleed air switches and pack switches).

Oh, and it wasn´t the TO config warning, this can only sound on the ground (or to be more precise while the air/ground switch is sending a ground signal).

Edited May 13, 2023 by Litjan

[martinlest]

If it wasn't the TO warning (it sounded the same) then I have no idea what it could have been.

Yes, next time I will pause the sim and take some screenshots. I haven't had any issues before this week (and I have had the 733 for some years now!). Odd.

Thanks.

[Litjan]

The problem in the 737 warning system design (that we have faithfully reproduced) is that the warning horn for the takeoff configuration and the warning horn for excessive cabin altitude (it being >10.000 feet i.e. not enough pressure to breath) is totally identical.

Boeing reasoned that pilots are smart enough to figure out that if the warning horn sounds on the ground, it can´t be the cabin altitude warning and if it sounds in the air, it can´t be the takeoff configuration warning.

The Helios pilots proved them wrong.

After that incident they implemented a new Abnormal Checklist for this, it is called "warning horn intermittent" (or so). It goes like this:

If you hear an intermittent warning horn, find out if you are

A.) on the ground  - this means you are hearing the takeoff configuration warning horn, do not take off!

B.) in the air - this means that you are hearing the excessive cabin altitude warning horn, do the "Excessive Cabin Altiude Abnormal Checklist".

 

The vast majority of pressurization problems are due to forgetting to turn on the packs, either after engine start (where you turn them off to have enough bleed pressure to start the engines) or after takeoff (if you took off without packs operating to squeeze out some more power from your engines).