"Position error" of outflow valve B737-300/-500 (4th altimeter)

[Badente]

A little experiment from the last sim-session:

Problem: A B737-300 in IMC. Loss of all air data. Bare CAT I weather. Now bring her down.

Basic flying is no problem thanks to pitch & power. Biggest concern is altitude. VSI is operative (source: IRS) but you only get a rate. The ground echos from the weatherradar work quite well for a crude altitude readout at higher levels (tested in real life). But closer to the ground a more exact, preferably barometric, altitude is desirable.

We came up with the following: Below 10.000ft make the ship bleedless, dump the cabin pressure and make sure the outflow valve stays in its full open position. Then use the cabin altitude indicator as altimeter (not tested in real life - at least by me).

Is there anybody out there who can share some information on the position error of this improvised altimeter as the sim may not be entirely accurate in this respect. I couldn't find anything in my official documentation either. Any other ideas how to solve this problem?

Regards,
Badente

[Cough]

Flying with packs on and outflow valve open at FL80 indicates about 83 at 250 kt, but just over 90 at 330kt. Once low, fly slow to compensate.

(found when positioning an aircraft back to base with an inop outflow valve which was secured open)

[hetfield]

There is another thing you have to think about.

Is the cabin alt indicator on 737 referring to 1013hp?
If not, you have to apply the local QNH correction.

Sorry not current on type.

[john_tullamarine]

If the ILS is available, the correct slope can be determined by pitch/attitude for nominal descent performance and the ILS flown as a flight with unreliable airspeed exercise to, say, cat 1.

We used to do this exercise regularly on endorsement training .. all pitot static instruments failed, radalt, etc so that there was nothing other than attitude and thrust and ILS/locators.

Invariably a great confidence builder.

[ab33t]

Pressure in the cabin wil be slightly lower than outside and higher speeds will effect this even more

[A37575]

We used to do this exercise regularly on endorsement training .. all pitot static instruments failed, radalt, etc so that there was nothing other than attitude and thrust and ILS/locators.

Wouldn't happen now. The emphasis in simulator recurrent and type rating training is strictly box ticking. No time permitted for perceived life skill training that you described. More's the pity.

[Badente]

Originally Posted by Cough

Flying with packs on and outflow valve open at FL80 indicates about 83 at 250 kt, but just over 90 at 330kt.

Some numbers from the field – this is exactly what I was hoping for. Thank you! I'll compare them next time in the sim. Would you mind sharing the series of the B737 you flew?

john_tullamarine: Do I understand you correctly that the focus of the training you mentioned is mainly on pitch & power flying? Having the VSI helps a lot in this case: ~500fpm and you can almost be sure you hit the right GP beam (assumed 3° glide and no other unusual conditions). How do you assure terrain clearance when you are not established on the ILS?

Originally Posted by A37575

The emphasis in simulator recurrent and type rating training is strictly box ticking.

I'm a low hour (~1.500h) F/O so I count myself lucky to have this opportunity during my refreshers. However the pressure to adapt our B737 syllabus (which allows for such things) to the syllabus of the "more computerized" aircrafts (which doesn't) is there. They see it as standardisation across the fleets which isn't necessarily bad. But in my opinion they put the emphasis on the wrong things. Besides the standard sim ops (engine failures, etc) I see the sim as a great opportunity to try escape strategies from abnormal situations outside the book without killing myself in the case they weren't as great as I tought. What do you think?

As a longtime reader of these forums I'm always amazed about the knowledge you people have. Thank you for contributing.

[Cough]

734....Cheers Cough

[john_tullamarine]

focus of the training you mentioned is mainly on pitch & power flying?

Certainly is, as an adjunct to the normal program of training .. ie a time available extra. The main aim is to get across the message that the newer aircraft is still an aircraft and the basics apply without HAVING to have all the bells and whistles singing their songs ..

Having the VSI helps a lot in this case

.. of course it does .. which is why we remove it for the exercise

How do you assure terrain clearance when you are not established on the ILS?

My post noted the use of ILS as part of the recovery. Comparison of flight with unreliable airspeed data (which is just normal flying if you don't live and die by the FD) with the ILS observations confirm that you are on the correct slope.

[Badente]

Thank you Cough.

Originally Posted by john_tullamarine

My post noted the use of ILS as part of the recovery.

Got your point. I was focussed on the getting-to-the-ILS-without-hitting-ground part. Think of the SOF/LBSF ILS RWY 27 when arriving from the west. Lots of high terrain around. I totally agree with you on the pitch & power slope crosscheck.