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The reasons why a clock is necessary for IFR have been explained elsewhere, but I haven't seen the reason that the clock has to have a sweeping second hand.

It seems like when timing things it would be more useful to have a ticking second hand, why is it expressly required that it be a sweep?

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  • $\begingroup$ From what I can find, a "sweep-second hand" means that it is concentric with the hour and minute hands, not that it has to have a certain motion. $\endgroup$ – fooot Jun 5 '17 at 15:00
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    $\begingroup$ Odd language. Does it have bristles attached to it, like a little broom? $\endgroup$ – Koyovis Jun 6 '17 at 8:29
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As @fooot mentions in the comments it means that the hand must be concentric with the hour and minute hands, it is not a reference between the difference of deadbeat and sweep seconds as many watch brands commonly reference now. This is a regulation that is intended to clarify the difference between a second hand that references the main dial and one that is on a sub dial. Historically planes may have had more complicated nav clocks like this hamilton where the large second hand is for the chronometer and not the elapsed time clock.

enter image description here

(source)

You can find what looks like one of the early AC's on digital clocks here. It references FAR 91.33 (which seems to no longer be in place) you can find the original verbiage here. I can not find any FAA legal interpretation that further defines "sweeping seconds".

On a bit of a side note, wrist watches (in any form) are not a legal substitute for a mounted clock.

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In IFR flight you sometimes need to accurately time seconds for approaches or turns.

Early watches did not have a "seconds" hand. Some watches had a small secondary dial for seconds, but because of the small size it was hard to read. The rule was written "sweep-second hand" meaning a seconds hand that swept around the main dial so that a pilot could accurately time seconds.

As "fooot" says above, it does not refer to the type of motion the hand makes.

I have seen some aircraft equipped with digital clocks and often wondered if I needed to wear a "sweep-second hand" watch to be legal.

Photo Source

Photo Source

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  • $\begingroup$ I have seen some aircraft equipped with digital clocks and often wondered if I needed to wear a "sweep-second hand" watch to be legal. Unless your watch is installed equipment on the aircraft, it still wouldn't matter. I believe (but don't know for sure) that would mean you add your watch to the aircraft's weight and balance data. $\endgroup$ – Steve V. Jun 5 '17 at 17:19
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    $\begingroup$ @SteveV. According to this legal interpretation from the FAA, wrist watches are not a legal substitute for a mounted clock. $\endgroup$ – Dave Jun 5 '17 at 17:34
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    $\begingroup$ But digital presentations are legal even without a second hand. $\endgroup$ – David Schwartz Jun 5 '17 at 19:27
  • $\begingroup$ Here is the ruling on using a watch (can't) but you can use an installed GPS or other Digital Clock. faa.gov/about/office_org/headquarters_offices/agc/… Another installed appliance with a permanent clock display that meets the requirements of § 91.205(d)(6) that displays "hours, minutes, and seconds with a sweep-second pointer or digital presentation," is adequate for the purposes ofthis regulation. $\endgroup$ – Canuk Jun 6 '17 at 6:15
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FWIW, local DPEs and the FAA FSDO agree that a digital clock with a seconds display may be used as an equivalent of an analog clock with a "sweep second" pointer. This was a big deal, perhaps in the 80's, but everyone has come to accept the digital clock as acceptable for flight operations and flight tests.

The point about a clock aircraft mounted is a MEL / 91.213 issue that should be understood by wanna-be instrument pilots. Practically, installed GPS satisfies the clock requirement.

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