B737NG PFD Barometric Indications

[Sir Stanley Bigh]

As I understand QNH/STD selection via the EFIS box on the NG:

If you have STD selected and then 'twist' the QNH knob (or barometric selector) in either direction, the preselected QNH (the white numbers) always starts from 1013. Can anyone confirm this or did I dream it? The only reference I can find in the systems manual is pasted below:


1 Barometric Settings (green)
Indicates the barometric setting in either inches of mercury (IN) or hectopascals (HPA) as selected on the EFIS control panel.

Display is boxed amber if numeric is set and airplane is climbing above transition altitude, or if STD is set and descending below transition flight level.


2 Preselected Barometric Setting (white)
STD is displayed when the Barometric Standard (STD) switch is selected on the EFIS control panel.

When STD is displayed, a barometric setting can be preselected on the EFIS
control panel barometric selector and is displayed in small white characters below STD.

[Radar Love]

If you have STD selected and then 'twist' the QNH knob (or barometric selector) in either direction, the preselected QNH (the white numbers) always starts from 1013.

Yep, I've seen this too. Pre-selected QNH starts from 1013 when you twist the knob with STD selected. At first you'd think it started from the previously set QNH, but it probably makes more sense to start from standard...origin and dest can be huge distances apart :-)

[BOAC]

To answer your question directly, Sir Stanley, yes, that is how is is designed. Presumably less chance of Mr Up-cock if the 'default' setting is the one you expect to be using (e 1013.2/29.92) when you are actually flying with that set. If a 'random' QNH setting (eg the last one used) was allowed, there is always the possibility it might not be noticed, whereas 1013 does tend to be 'noticed'. Additionally if an inadvertent selection of QNH is made above TL there will be no 'false' input to the autoflight system.