Why is it that when I edit the descend speed from ECON to 300, sometimes one side, sometimes both sides show the new FMC speed as 299 above the speed tape in the PFD.
I know pushing speed intervene twice fixes this and both sides now will show the correct edited speed.
I do not know why this is. Maybe someone here does.
Oops, that would be on the 737 NG.
Roj approved
9th Jun 2017, 00:39
Is 300 to close to Vmo at that FL?
On the bigger Boeing at TopD (FL410) folks enter .85/320 and as it rolls over it will indicate .836 max until you descend a few thousand feet then it'll creep up to .85.
The logic won't let it indicate closer than 16kts from Mmo/Vmo.
No, 300 is nowhere close to a limit speed. It's just oddly off by one knot once in a while.
flyby797
10th Jun 2017, 19:40
It is the case on B737NG indeed and i Ve allo always wondered why. If anyone has the answer?
B737C525
11th Jun 2017, 20:21
When you enter that value on the CDU, it sets in train an interesting set of events, which are generally kept from the pilot's perception.
People have the impression that the 737-NG has a digital FMC system. In fact, it retains the -EG system, itself only a small step forward from that fitted to the -100/-200 variants.
It is a first-generation electro-mechanical system with some electro-magnetic elements, relying heavily upon technology developed as a sideline by Robert Moog.
Mr Moog, fascinated by the beauty of aviation and the graceful manner in which planes swooped and circled his home, realised that there were many parallels between the musical art which he was making electronic reality with his synthesisers, and the science which made jet-age flight possible.
He engaged with the US military, selling a number of unique benefits which applying early electronics to aviation's challenges offered, even when integrated circuits and micro-processors were real options. For example, while a digital parameter can change by an enormous amount in an instant, a system reliant for its high-order functions on low-speed DC motors, tensioned springs, and aneroid capsules connected to witness arms, and for secondary processing on multiplicitary indexing counterscales, and rate-based graticulators, always offers a smooth transition, regardless of malfunction.
Similarly, given the extraordinary rate of crashes in testing at the time, using mechanical devices allowed investigators to see what had been where, when things went wrong (or at least, at the ensuing moment of impact).
Short-term power outages, such as when the pilot moves the toggle switch to create the satisfying clunk as a generator goes on bus, have little effect on systems whose maximum rate of change is, in modern terms, glacial.
The Boeing Commercial Airplane Company, of course, was delighted to retain a system which, like the rest of the 737-NG, appeared new, whilst in fact offering none of the potential benefits that technological development could deliver, and ordered the Moog equipment as standard fit for the new model.
To return to the original question, it's likely that the winding wensiometer on the second-order speed-knob sensimotor has been set too close to the engrimetric limit. This is difficult to spot in maintenance, but if you ask your line guys to change out the whole unit, it can be sent for shop overhaul. In the short term, a dab of vaseline on the flangling arm may buy you a few days grace, but this is not an approved procedure.
Double-pushing speed intervene simply releases all the elastic pre-tension in the abliasperic system and allows the idler pulley fleuribond bias to travel back to its datum setting; as you identified, it's a quick fix, not a long-term solution.
Now that was funny ( and probably does fix it, too)