Scales and Bezels: The Complete Flight Tools Guide

THE ROTATING BEZEL: A TOOL BORN IN AVIATION

The rotating bezel - that external rotating ring found on so many watches - was born in aeronautics, from the hands of an American navigator.

In 1929, Lieutenant Commander Philip Van Horn Weems, a US Navy officer and central figure in aerial and celestial navigation of his era, filed a patent for a watch with a rotating inner dial coupled to an external rotating bezel. Weems had trained Charles Lindbergh in astronomical navigation techniques before and after the historic transatlantic crossing.
His partnership with Longines - official timekeeper of the 1927 transatlantic flight - resulted, in 1931, in the Longines Hour Angle Watch: the first watch in history to integrate a rotating bezel as a navigation tool. The patent was only granted in 1935.


Credit: Collectorsquare.com

This bezel was graduated in hour angle degrees - too complex for most pilots. Later versions simplified it to a 0–60 minute arc, alignable to the second against the seconds hand or a radio time signal. The principle was established: the bezel as a time measurement tool, no longer a time-setting aid.

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Around 1935, Heuer paired the first rotating bezel with a wrist chronograph, once again for aviation purposes.

Credit: Grail-watch.com

In the years that followed, Hanhart and Tutima (whose history is detailed in our article Pilot's Chronograph: the history of watches that conquered the skies) integrated this bidirectional single-marker bezel into production pilot chronographs made for the Luftwaffe during the Second World War. A simple metal dot or painted marker: nothing more.

These watches, stripped down by functional necessity, would define the aesthetic and functional DNA of the pilot's chronos for generations to come.

BIDIRECTIONAL IN AVIATION, UNIDIRECTIONAL IN DIVING

The question often arises: why do dive watches have a bezel that only turns one way, while pilot's watches do not?

The answer is a matter of safety - but it only applies to diving.

The ISO 6425 standard mandates unidirectionality for a precise reason: if a diver's bezel is accidentally knocked, it can only rotate in the direction that reduces the displayed time. The diver believes they have spent more time underwater than they actually have, and surfaces earlier with a sufficient oxygen reserve.
This is the conservative error: too much air in the tanks at the surface is far better than too little at depth. A rotation in the other direction would give the diver an indication greater than their actual oxygen reserve, which could be fatal.

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In aviation, this directional constraint does not exist. Flight procedures require rotation in both directions. Marking the start of a holding pattern leg may require turning backwards if the cue has already passed. Anticipating a future departure requires turning forward. Repositioning a marker mid-procedure may require either direction depending on context.
The bidirectionality of the bezel, as on the DÉDALE Waypoint, meets the pilot's functional needs.

FLYING BY HEADING AND CLOCK: NAVIGATION IN ITS FUNDAMENTALS

Before detailing the uses of the bezel, a fundamental point that every instructor teaches from the very first hours of Private Pilot Licence (PPL) training: heading and time navigation.

This method is the oldest, most reliable and most universal aerial navigation technique. It requires no GPS, no VOR, no avionics whatsoever. A compass, an estimated ground speed, a watch. The rest is arithmetic.

The principle: before the flight, you determine the magnetic heading to hold and the ground speed (true airspeed corrected for wind drift). You calculate the duration of each leg. You identify the waypoints. In flight, you hold the heading and note the time. The chronograph then becomes a tool for this final step.

This method still underpins PPL training in virtually every country today.

Among the most iconic navigation watches are the "Beobachtungsuhr", better known by the abbreviation B-Uhr, in Type A (1936) and Type B (1941) versions, introduced for the Luftwaffe.

They were intended for navigators, not pilots.


Type A - Credit: acollectedman.com / Type B - Credit: bba-watch.com

THE BEZEL AS A TOOL: PRACTICAL USES IN FLIGHT

The rotating bezel comes into play in many concrete procedures. Here are a few examples:


Time estimates: Checking and validating flight times between each waypoint. Marking revised estimates in the event of a diversion.

Fuel management: Marking the takeoff time or the calculated time limit before the flight remains standard procedure. Based on the aircraft's known fuel consumption, this allows comparison with the gauge reading and identification of any discrepancy (overconsumption, leak, faulty gauge, etc.).

Time reference: Aligning the bezel with the takeoff time to instantly calculate the current flight duration, without mental arithmetic. Useful for any pilot logging flight hours in their logbook, as well as for aircraft maintenance. Also practical at flying clubs, where aircraft rental is typically charged by the hour.


Military use: A lesser-known use of the rotating bezel combined with the chronograph: bomb release point calculation in wartime.

The principle is straightforward. In bombing navigation, the pilot or navigator calculates before the flight the time from the Initial Point (IP) to the target at a known ground speed. Over the IP, the chronograph is started and the bezel aligned. When the hand reaches the calculated marker, the bombs are released. The Omega CK2129 - official watch of the RAF - was used in this way: its lockable bezel, secured by a second crown, protected the marker from accidental displacement during formation manoeuvres.

Specialised watches known as bomb timers or return-to-zero chronographs (ritorno) were also developed by several manufacturers (Universal Genève, Minerva, Zenith, Breitling) for Allied and Axis air forces. Their single hand rotated in reverse from a preset mark down to zero. The chronograph and bezel on a standard watch fulfilled the same function with a single instrument.

BEZEL FORMATS ON PILOT'S WATCHES

Not all bezels are equal, and their graduation immediately reveals the philosophy of their designer.

0-60 minute bezel: Sixty divisions, one per minute. Compatible with all flight procedures, all durations under one hour. This is the format adopted by the vast majority of pilot's watches since the 1940s.


0-12 hour bezel: Two main functions: timing time differences measured in hours rather than minutes; and for transmeridian flights, some watches feature a bezel or inner ring graduated over 12 hours allowing simultaneous reading of two time zones. (On a watch with a 12-hour dial, an additional 24-hour graduation will necessarily be accompanied by a second hour hand, making it a complication known as GMT, UTC or Zulu.)

Countdown: Some aviation watches feature a bezel or complication allowing countdown timing from a preset duration. Used for time estimates, holding procedures or arrival window calculations. The bomb timer mentioned above is an extreme version of this.


Worldtimer: It plays a central functional role by enabling simultaneous reading of multiple time zones. It is generally engraved with a ring of reference cities, each corresponding to a standard meridian (usually based on the GMT/UTC/Zulu system). This bezel, whether rotating or fixed, works in conjunction with a 24-hour indication to distinguish between day and night zones.


Bezel with single marker (arrow or dot): The most universal, most understated and most philosophically honest version. No imposed graduation - the pilot decides what the marker means. A historical solution, adopted by the DÉDALE Waypoint WP-01.


Bezel with slide rule: By its very design, it is a true embedded computing device. Consisting of two complementary logarithmic rings, it is the most technically complex, which warrants a thorough analysis.

THE CIRCULAR SLIDE RULE

If the rotating bezel is an almost inseparable companion to the pilot's chronograph, the circular slide rule is its pinnacle. This is not decoration: it is the most powerful instrument that can be integrated on a watch dial. It is also the most difficult to grasp.



Credit: issuu.com

A mathematical logic: the logarithm

The slide rule is based on the logarithmic representation of numbers: values are not distributed linearly, but proportionally to their logarithm. The spaces between numbers narrow as values increase. This is not an aesthetic accident - it is what allows, by aligning two graduations, multiplications and divisions to be performed without mental calculation.
The principle traces back to John Napier's work on logarithms (1614) and William Oughtred's slide rule (1632) - three centuries before Mimo installed it on the Mimo-Loga, the first wristwatch to integrate a slide rule, which appeared in 1940/1941.
Breitling introduced the slide rule on the Chronomat in 1942, but it was in 1952 that this feature, once adapted for aeronautics, elevated the Navitimer to icon status.


Crédit : Watchprosite.com

The slide rule in flight

For the general aviation pilot preparing their flight at the briefing table - E6B in hand, ICAO chart unfolded - these calculations are everyday occurrences:

Speed / distance / time / fuel consumption : Knowing any two of the three variables, the third is obtained instantly.

Climb and descent rates : To plan the top of descent from cruise level, without GPS, as pilots who fly "by the seat of their pants" still do today.

Unit conversions : In French general aviation, altitudes in feet, distances in NM, pressures in hPa, fuel consumption sometimes in US gallons. The slide rule handles all these conversions in a single alignment.

True airspeed (TAS) : On certain models, altitude and temperature corrections allow estimation of the True Air Speed from the indicated airspeed.

OTHER SCALES

Tachymeter scale

The tachymeter is a graduated scale used to calculate an average speed from a measured time, based on the fundamental formula: Speed = Distance / Time.

In watchmaking, two main configurations exist.

On a rotating bezel: the logarithmic scale of the circular slide rule is used. Its advantage is versatility: applicable to any moving object, usable in flight, and requiring no chronograph in progress - the bezel is positioned manually. Its drawback: at least two of the three variables in the formula must be known.

Fixed (bezel, inner ring or dial): this is the most common form. Born from the automotive world, it relies on a fixed reference distance, usually 1 kilometer (known as "base 1000"). Its use is immediate: start the chronograph at the beginning, stop it 1 km later, and read the average speed directly. In return, it is entirely dependent on the chronograph and it is used over a maximum duration of 60 seconds.

In aviation, this second type is unusable as-is, for three main compounding reasons: aeronautics operates in knots, not km/h (or in extremely rare cases), flight times are very rarely less than 60 seconds and it is impossible to identify a precise kilometer marker in flight.

A fixed-base tachymeter is an indicator designed to measure speed over a fixed and identifiable distance - a function specific to the automotive world, whose needs differ from those that define a pilot's watch.

Telemeter scale

Dependent on a time trigger. Not relevant on a rotating bezel.

The telemeter exploits the speed differential between light (instantaneous) and sound (~340 m/s at 15°C). By timing the gap between a flash and its corresponding sound, one obtains: Distance (km) = time (seconds) × 0.340

The scale is non-linear, typically graduated from 2 to 20 km. It appeared on pocket chronographs as early as 1900-1910, adopted by artillery officers during the First World War to estimate distances to shell impacts. Popular throughout the 1930s-1950s.

Its connection to aeronautics is real but indirect: aerial observers and forward air controllers during the Second World War used it from low-altitude observation aircraft to estimate distances to ground targets. In flight, it is unusable: the aircraft's drift, speed and sound propagation conditions at altitude all invalidate the graduation.

Unlike the tachymeter - born of automotive culture with no aeronautical history whatsoever - the telemeter has a genuine military connection and a marginal but honest meteorological application. Its presence on a vintage chronograph is culturally legitimate. It is not a cockpit tool, however.

Pulsometer scale

Dependent on a time trigger. Not relevant on a rotating bezel.

Measures heart rate by timing a defined number of beats. Medical and sporting use. No direct aeronautical application - but often found on vintage multi-function chronographs from the 1930s–1960s.

Asthmometer scale

Dependent on a time trigger. Not relevant on a rotating bezel.

Determines the number of breaths per minute - scale typically graduated at 15, 20 or 25 breaths. Medical use.

Compass bezel (N/S/E/W)

South can easily be found with any analogue watch, and the other cardinal points deduced from it:
Set the watch to solar time (GMT / UTC / Zulu time)
Then point the hour hand towards the sun.
Form an angle between the hour hand and the 12 o'clock axis.
Bisect this angle - that direction is South.

The compass rotating bezel will help you easily read the cardinal points, after finding South, at that precise moment only. Limited but legitimate use for quick orientation on the ground. In flight conditions, its use is very limited.

Always going further

The beauty of watchmaking combined with its boundless creativity allows for an ever more versatile, specific - or even playful - adaptation of scales.
A few examples: double rotating bezel, double scale, decompression stops, maximum working depth duration, egg cooking times, etc.

SUMMARY TABLE

DÉDALE WAYPOINT

PILOT YOUR TIME

Frequently asked questions