View Full Version : Symbols on the 737 MCP Knobs.
B737NG_Pilot
2nd Jul 2017, 11:27
What is the significance of the symbols on the MCP Knobs.
Also why does the symbol on the CRS always opposite to each other... (Capt & First Officer).
eckhard
2nd Jul 2017, 14:28
They are symbols which resemble the 'bugs' on the associated instrument. On the original electromechanical-fitted 737-300 (and some later -200s) the HSI had a heading bug like a triangle with a slot in it. It also had a course line which had a top like a dagger. The ASI had a bug like the nib of a fountain pen (remember them?). The symbols on the MCP knobs match those, in an attempt to remind the pilot which parameter they are trying to control. (The bugs on the newer EFIS-equipped machines may not be identical to those on the MCP. An interesting historical hang-over.)
Despite that, and the fact that they are shaped differently and feel different when they click around, I and others have selected the wrong knob and then watched the wrong instrument, confused, as the aircraft does not respond as expected. Like the person who thinks that the lift will arrive sooner if he pushes the call button multiple times, I and others have then moved the offending knob even further, in an attempt to get the stupid aircraft to behave!
I remember one particular night accelerating towards Mount Teide at TFS instead of turning right towards the ILS LOC. How we laughed....
As far as the symbols on the F/O's and Capt's CRS knobs being opposite to each other is concerned, I suspect it is just the way they have been mounted. I don't know of any reason why it would matter.
B737NG_Pilot
4th Jul 2017, 04:46
eckhard thanks a detailed reply. It clarifies quite a few things.
Though, there seems to be some logic as to why F/O's and Capt's CRS knobs are opposite to each other.... It was one of the questions asked during a command viva.
External light switches can have different shapes as well.
framer
4th Jul 2017, 10:10
I do wonder about the four knobbly ones.....any ideas?
eckhard
4th Jul 2017, 10:22
I think the knobs or bumps are supposed to represent light-bulbs. There was an effort in the US (post-WW II?) to standardise the shapes of cockpit controls and switches. Hence: the gear lever with a little wheel on it; the flap lever shaped like a flap; engine controls with standard shapes/colours (straight black throttles, blue wavy propellers and red spiky mixtures).
The Brits took a while to catch on. Typical controls included 'plungers' or push buttons for the gear and plain knobs or switches for the flaps.