MCP Speed Display 1 Digit Off of Speed Display on PFD

[KORDATC 80]

I was flying the -400F Korean Air when I realized the MCP settings were constantly 1 knot shy of what was being displayed on the PFD.

 

 

Now, grant it, I was just flying for fun around SFO and did not set up the FMS or anything before the flight including weights or anything.  But the FMC shouldn't affect the relationship between the two displays should it?

 

Regards,

Rob

[scandinavian13 5,192]

That's weird. Absent any steps to reproduce, however, I'm hesitant to go chasing after it. I've never seen this come up before. Sure there wasn't anything interfering with your sim session?

[KORDATC 80]

Not that I know of. Like I said, I really wasn't trying to do everything by the book. I just loaded up the KAL -400F at SFO. I set the fuel to I think 15% or 20%, set the flaps to 10 set the lights and autobrake to RTO. Then I set the airspeed to 250 initially on the MCP. I took off with TOGA and then selected the THR switch on the MCP to bring it to CLB thrust. I then selected the SPD switch to keep it in speed mode. I just did a loop around the bay. It wasn't until I was slowing down over Oakland that I noticed the speed indications didn't match. I will try again when I get home tonight to see if I can reproduce.

[KORDATC 80]

Alright, tried reproducing and couldn't. 

[Qavion2 39]

Strange.... I think I've seen this in real life.  I can't remember what was broken though (MCP or Flight Control Computer).

 

Then there are times I push the wrong button and see this...

 

[77west 139]

Chalk it up to the vagaries of computers? I would not generally be worried about a couple knots difference between displays, except that the selected IAS should really be driven off the MCP and thus match. But then this is an old airplane, the newest 744 is still around 10 years old now, so perhaps just the effects of age on the systems...

[Qavion2 39]

Chalk it up to the vagaries of computers?

 

 

Indeed. I once read a memo relating to "Neutron Single Event Upsets" (NSEU's for short). High energy (solar?) particles are constantly bombarding the earth and these can go flying through electronic circuits and change their "1" or "0" states (especially in the thinner air at higher altitudes). The memo advised me that if the MCP was showing specific (mis-)behaviour, that it could be due to NSEUs. The subject is highly contentious, but it was an official Boeing memo. I recall the memo just advised to cycle the MCP circuit breakers and check every single increment on the MCP knobs

 

I wish I'd kept the memo... I've had trouble explaining to pilots in the past (They must have thought I was part of the tinfoil hat brigade)

 

Note that the target airspeed/mach on the PFD is not sent directly from the MCP, but goes through the FCCs (Flight Control Computers). I recall that if you pull the FCC circuit breakers quite a lot of the information on the PFD disappears.

 

Cheers

JHW