Time and aviation are inexorably entwined, but aviation reckons time in ways strange to those whose feet are always planted on the ground. From the moment we begin flight training, our experience in the air is measured by time. Instructors fill out our logbook documenting training measured in six-minute intervals converted to tenths of an hour. We may be credited with 1.3 hours of dual instruction -- the time in the aircraft from engine start to engine stop. That means we flew more than one hour and 12 minutes, but equal to or less than one hour and 18 minutes.
Most aircraft have a Hobbs meter, an electronic timing device that starts when electric power from the battery is applied to an aircraft, or it may be attached to an oil pressure switch measuring engine running time only. The Hobbs "taxi meter" determines how much it costs to fly (see "On the Clock," November 2003 AOPA Flight Training). An apocryphal student, asked what instrument failure was the most dreaded, answered, "A runaway Hobbs meter."
A Hobbs meter may be activated by a "squat switch," in which time is measured only when the airplane is flying with weight off the wheels. Taxi and ground delays are not counted. Such a configuration advantageously measures time between engine overhauls, inspections, and replacement of time-limited components. The aircraft you rent must be inspected every 100 hours of time in service, which usually is determined by a recording tachometer or a Hobbs meter.
Most piston aircraft have a tachometer that seemingly measures time, but this is only accurate at cruise rpm. Tachometer time is slow when idling. Pilots often debate which airplane is most economical to rent, judging cost per hour and how the FBO measures time.
Coordinating time
As we become more experienced, time remains the name of the flying game. Of course, we fly to save time, and even the slowest training airplane is twice as fast as an automobile.
However, when travel was less common and even slower, each locality set its own time standard. It did not matter if each town set its clocks differently. The 24 time zones were inaugurated to make railway timetables sensible. Time zones are labeled in a strange fashion. Beginning to the east of Greenwich, England, zones were designated in alphabetic sequence eastward from A (omitting J) to M. Then the zones are labeled beginning to the west of the Greenwich zone with the letters N though Y. The Eastern United States time zone is R, and the Pacific time zone is U. The zone that straddles the International Date Line is split in half, with M to the west and Y to the east. That leaves only Z as the label for the time zone straddling the prime meridian and Greenwich, England.
These labels are rarely used now, except for Z. Z or Zulu time is synonymous with Greenwich Mean Time (GMT) and Coordinated Universal Time (UTC).
The global standard Greenwich Mean Time was used to synchronize faster travel and worldwide communication. This is defined as the mean solar time at the prime meridian -- "mean" because it was an average of the Earth's non-uniform rotation. Day lengths vary plus or minus 15 minutes because of eccentricity of the Earth's orbit.
GMT was the standard until the innovation of the atomic clock that permitted more accurate timekeeping. Now all aviation times are technically UTC, expressed using the 24-hour clock. The transposition of the second and third letters in the abbreviation (UTC instead of CUT) is thanks to the French Bureau International de l'Heure, which decides whether a leap second or two must be added at the end of each year to keep time coordinated with the Earth's variable rotation.
For practical piloting purposes, the difference between GMT and UTC is inconsequential, but not for the mathematics GPS receivers require to determine accurate geographic location. That is an important hint. The most accurate clock in an aircraft is the GPS. There usually is a page where UTC can be displayed, and that value can be used to set an aircraft clock and your wristwatch.
Government rounding
The FAA typically rounds time to the nearest minute with the change to the next minute at 30 seconds. However, if you do not have a GPS and ask for a "time check" from any flight service station, control tower, or air traffic control facility, the answer will be expressed in hours, minutes, and quarters of a minute. (Yes, they also round up to the next quarter of a minute after eight seconds of each quarter.)
"Long Beach Ground, November Three-Four-Seven-Golf-Sierra, say time." The controller consults one of several large digital displays provided in all FAA facilities and replies, "Time now one nine five seven and three-fourths," or "The time is one nine five eight Z."
A controller may express time simply as minutes, omitting the hours, when no misunderstanding would occur. "November Two Three Four Five Golf, expect further clearance at one eight past the hour." A controller may occasionally refer to local time, but then the word "local" is appended to the communication.
Converting time
Converting local (wristwatch) time to UTC (GMT or Zulu time) takes two steps. Local time must be converted to a 24-hour clock, and then to UTC expressed in four digits. The first step is easy; just add 12 hours to any time after the noon hour. Thus, 7 a.m. is 0700 (note the leading zero), and 3 p.m. is 1500. By convention midnight is 0000, and not 2400. This time based on 24 hours is sometimes labeled military time.
Next convert 24-hour time to the time over the prime meridian by adjusting for the number of time zones between your location and Greenwich, England. As the sun rises in the East, local time any place in the United States is later than Greenwich time. A way to recall is to visualize the sun directly over England at their noon, and then observe the sun is still well to our east. Therefore, local time in the United States is earlier or behind Greenwich time. That also means our aviation days end before local midnight. The exact difference depends upon the time zone -- Eastern, Central, Mountain, Pacific -- and whether standard or daylight savings times are in effect. Thus, you must add to local time to convert to UTC.
Time in your tanks
Flight planning is governed as much by time as it is by distance. The vagaries of winds aloft will determine your maximum cruise range on any given day. But fuel flow in cruise flight is constant, and we can determine almost to the minute when we will run out of fuel. This is why pilots best compute fuel as time. When filling out a flight plan, note that the box marked "Fuel on board" does not ask for gallons, but hours and minutes. A safe pilot always lands with one hour of fuel aboard, whether or not he has reached the intended destination. |
A trick with a conventional wristwatch may help. Let us say your watch is set to Eastern Standard Time and reads 3:17 in the afternoon. UTC is five hours earlier. Count backwards from the 12 five hours (the amount that must be added). Place your finger opposite the 7, and rotate the watch so 7 is upright. Consider that the new "12," and read the hour hand relative to that new 12. The answer should be 8. However, there are two possibilities, 8 and 20 (adding 12). The ambiguity is resolved by noting which side of midnight it is in England. In this case the UTC hour is 20. As minutes remain the same in all time zones, the answer to the conversion is 2017 Z.
You've probably noticed the many watches for pilots advertised in aviation catalogs. Most of these have a rotating bezel to help simplify UTC conversions, or a digital readout that you might set to UTC while the hands display local time.
Defining 'night'
The exact times in which night and day occur are important to pilots, because many rules change with the absence of the sun. The FAA definition of night is not sunset and sunrise, because the Earth remains lighted for some time after the sun has passed below the horizon. Rather the FAA chose civil twilight to define night -- not to be confused with "uncivil" twilight, I suppose (see "Legal Briefing: Defining 'Night,'" November 2004 AOPA Flight Training). Civil twilight is further defined as the exact time the sun is 6 degrees below the horizon. To put that in perspective, as the sun's disc subtends a half-degree, the sun is 12 times its size below the horizon at civil twilight. However, it is still not entirely dark then, as about 18 degrees are required before the sun provides absolutely no illumination. (This is called astronomical twilight, and it has no official aviation relevance.)
The time it takes the sun to reach 6 degrees below the horizon after sunset depends on latitude. Near the equator, the sun drops vertically and takes only 24 minutes after sunset to reach 6 degrees (as sun motion is 15 degrees per hour), and near the poles the sun never sets during half the year. So the exact time of civil twilight depends upon latitude, but also the longitude. Even within a single time zone, night varies plus or minus 30 minutes.
We instinctively understand that summer days are longer, but the time between sunset and civil twilight is longer as the sun approaches the horizon more acutely during summer.
Unfortunately the FAA is not consistent with the official definition of night. Civil twilight is applied to logging night flights and training, VFR minimums, and most aircraft lighting requirements. Night VFR fuel reserve requirements are 45 minutes, compared with 30 minutes for day VFR (FAR 91.151).
However, a more prosaic night definition of sunset to sunrise governs when position lights (red, green, and white) must be on as well as the requirements of a pilot requesting a Special VFR clearance. Between sunset and sunrise such a pilot is required to be qualified and current on instruments, and the aircraft must be equipped for instrument flight (FAR 91.157[b][4]).
Remember one other important definition of night -- night currency to permit the transport of passengers is based upon the definition of night as one hour after sunset and before sunrise (FAR 61.57[b]).
Best advice
The best advice is to use sunset to sunrise to define night for operational considerations, and thus err on the side of safety. Similarly, logging your night flight time based on one hour after sunset to one hour before sunrise is the most conservative approach. Keep in mind that many GPS units provide accurate sunrise and sunset times. In this case, too, the GPS is our best friend for time.
Dr. Ian Blair Fries is a CFI, senior aviation medical examiner, and ATP, and holds a Lear 35 type rating. He serves on the AOPA Air Safety Foundation Board of Visitors and is cochairman of the AOPA Board of Medical Advisors.
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