Autopilot IAS/MACH display

[jornov1 0]

Hi Friends:

I would like to know what the letters that flash in front of the selected speed with the IAS / MACH dial on the autopilot panel mean. I could not find this. Now I have "A" flashing in front of .78 M. Flying at FL380 and .78 Mach speed. I can`t see any wrong.

Thank you

Jorge Novelo

 

 

Edited July 4, 2017 by jornov1
Correction

[scandinavian13 5,192]

1 hour ago, jornov1 said:

I would like to know what the letters that flash in front of the selected speed with the IAS / MACH dial on the autopilot panel mean. I could not find this. Now I have "A" flashing in front of .78 M. Flying at FL380 and .78 Mach speed. I can`t see any wrong.

I believe it's either 'A'bove or 'B'elow your max or min speed.

[Chock 17,769]

If it flashes an A, that means you are under speed, if it flashes an 8, that means you are over speed for either the Vmo/Mmo limit, the landing gear limit or the flap limit. The reason it flashes, is to attract your attention.

There are some limitations on what speeds you can actually select on the MCP which inhibit going over the Mach limit, but it is possible to select speeds above the flap and gear limits etc, so as with everything, it is possible to put stupid settings in the thing, hence the warning annunciators. Since the 737 can fly at Mach .82 (in fact it can fly at Mach .87 although you shouldn't), whilst you were at 38,000 feet it was telling you that you were 'only' at Mach .78, which is why you were getting the flashing A, although you can ignore it if you like, since Mach .78 is actually quite a common speed for the 737 to cruise at.

Here's a useful clickable interactive study aid for the MCP:

http://www.b737studyguide.com/B737NGiPad/B737/MCP/Honeywell/MCP.html

[jornov 0]

It has been perfectly clear to me. I appreciate your information and the link.

Greetings from Mexico

Jorge Novelo

[vc10man 732]

Once more, two sages of sound advice, Kyle+ Alan, to the fore with logical explanations. Kudos to both.

[JoeDiamond 67]

22 hours ago, Chock said:

although you can ignore it if you like, since Mach .78 is actually quite a common speed for the 737 to cruise at.

While .78 is a completely normal cruising speed at FL380 I wouldn't ignore the indication because something else isn't right.  You shouldn't see that during cruise and you should figure out whats wrong.  My initial guess would be you have the aircraft weight wrong in the FMC.

[jornov 0]

I agree, that is why a notice should never be ignored. In fact some other parameter must be incorrect. For that reason each message as insignificant must be well understood, in that measure you will have a safe flight.