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According to FAR 91.205 (d)(6), one instrument required for IFR is:

A clock displaying hours, minutes, and seconds with a sweep-second pointer or digital presentation.

Why is it necessary to time things in the cockpit, or know the time?

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    $\begingroup$ In Australia at least, a timepiece is required for VFR flight too. $\endgroup$ – Ben Nov 17 '14 at 8:33
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There are several reasons:

  • For departure clearances. An IFR clearance may have a void or release time so that clearance is only valid between certain times.
  • For holding pattern legs. A standard holding pattern is based on a time of one minute at or below 14,000' MSL and 1.5 minutes above 14,000' MSL on the inbound leg.
  • For holding pattern clearances. Holding instructions include an EFC (expect further clearance) time, so that if you have a communication failure while holding then you know when you can safely leave the hold and start an approach.
  • For timed approaches, where an aircraft is cleared to start an approach at a specific time
  • For timed turns. A standard rate turn is 360 degrees in 2 minutes, so with only a timer and a turn coordinator you can turn a specific number of degrees.
  • For en route reporting and general time/distance calculations. Especially in a non-radar environment (i.e. when ATC can't see you on radar) you may be required to give ATC actual or estimated times for passing certain fixes. (This is not strictly IFR only, but it's still relevant.)

All of these scenarios are discussed in detail in the FAA's Instrument Flying Handbook and in the AIM.

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    $\begingroup$ Bonus points for digging up the link to the instrument flying handbook (which I was too lazy to do) :) $\endgroup$ – voretaq7 Nov 14 '14 at 20:29
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In IFR the pilot needs to have a second-accurate time indicator for a variety of maneuvers.

For example, the standard holding pattern is 1 minute turns and 1 minute straight legs. Having the number of seconds allows the pilot to fly these maneuvers accurately.

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When flying under IFR in IMC you have no external visual references, so there are a number of reasons you may need to time something:

  • Some approaches require you to be able to time them.
    VOR approaches have a table of times from the final approach fix to the missed approach point at a given groundspeed: Since you have no other way to judge distance over the ground you must use the clock.
  • A "normal" holding pattern consists of one-minute legs with one minute (180-degree) turns at each end.
  • Turns under IFR are assumed to be made at "standard rate" (3 degrees per second).
    If your DG fails you can make "timed turns" with the turn coordinator, a compass, and a clock (30 seconds for a 90-degree turn, one minute for a 180, 2 minutes for a 360) and come out pretty close to your intended course, without being thrown off by compass lead/lag or acceleration errors.

There may be others but these are the ones that spring immediately to mind.

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Some procedures require timing, e.g. holding over a fix or NAVAID is usually done in 1min legs. Precise timing is necessary here. Some clearances can also contain a time (e.g. release valid until, revised approach time, etc), where correct timing is necessary.

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