IAS anomaly ON PFD during VNAV

[B737NG_Pilot]

The target speed displayed (magenta) on the PFD is always 1 knot less than the TGT speed set in the DES page (FMC).

Example we have set TGT SPD 250 Knots on the descent page.... The target speed displayed on the PFD is 249 knots.

Any reason why the 1 knot difference ??

[bob777]

Wow..big problemo. How about hand flying a bit and not giving a **** about 1 knot ...you a pilot right..or an anal pushbutton operator?

[Chesty Morgan]

Don't know why it happens but if you deselect then reselect VNAV it then displays the correct speed.

[STBYRUD]

Rounding error... the VNAV (formula derived) speeds are converted to integers in different ways by the DEUs and for display in the CDU, its absolutely meaningless for flying the thing though

[ImbracableCrunk]

I've seen it many times. I'm not sure how it can be a rounding error if I overwrite the ECON speed and put in .78/280 for descent.

Maybe I need to put in .78/280.00.

[underfire]

Wasnt there something in the aircraft logic that limits the speed below 10K to 245 kts to avoid busting the 250 limit?

[Capt Claret]

Very helpful Bob. A great example of the camaraderie of pilots.

[stilton]

Bob's version of camaraderie is looking in the mirror.

[OK465]

For some odd reason a line from the Tom Hardy movie "The Drop" comes to mind.

"They never see you coming, do they Bob?"

738 default below 10M is 240. CI plays a role in what active speed is displayed regardless of manual entries, depending on the STAR restrictions.

[flash8]

Bob are you related to the departed (but not forgotten) 411A....?

[Derfred]

Yep, we have a few Bob's in our airline. Not many fortunately.

CI plays a role in what active speed is displayed regardless of manual entries, depending on the STAR restrictions.

I don't understand this statement. If I enter a fixed speed, what has CI to do with it? Could you elaborate?

[OK465]

I don't understand this statement. If I enter a fixed speed, what has CI to do with it? Could you elaborate?

Yes, upon further review, I don't understand it either, my statement is a dumb one. Just call me Bob.

The situation I was trying unsuccessfully to allude to is a CI descent after T/D where the active CI speed, for example is 279, but the first waypoint on the STAR is coded 280. The active speed will remain 279 approaching and crossing the waypoint when one might expect 280.

[Derfred]

No worries OK...