Aviator's watches: The Mark XI

This is the fourth part of a series. The first part "The birth of aviation and the modern wristwatch" can be found at this link.

Dead Reckoning. Before World War II commercial air traffic navigating by using fixed radio beacons, where available, but primarily, navigation by sight using landmarks was still practiced. With the start of World War II the fixed radio beacons were turned off. British Bomber Command operated primarily at night, so navigation by sight was also useless. The only way for British Bomber Command to navigate over enemy territory, the used dead reckoning.

RAF bomber over Hamburg. This picture is from Wikipedia Commons.

Dead reckoning calculates the flight distance using speed and flight time, which, together with starting point and a direction measured by a compass, should theoretically give the exact position of the aircraft. In practice, the effects of side, head and tail winds resulted in dead reckoning giving an inexact position because the actual speed, distance and direction of travel is compromised. In addition, new sextants were developed allowing Allied pilots to use astro navigation. Astro navigation, was determining position with reference to the stars.

Both dead reckoning and astro navigation required accurate timekeeping. With aerial navigation being similar to maritime navigation, marine chronometers were used. However, marine chronometers did not stand up well to the continuous vibration of the propeller driven aircraft.

The Royal Air Force (RAF) purchased several thousand watches of the Mk VII and Mk VIII type , but these had problems in wartime conditions. These watches did not resist the salty air of the North Sea nor the humid climate of the Pacific.

In the early 1940's the British Ministry of Defense put out a specification set for rugged military watches that it wanted to supply its troops. The watches that met these specification were engraved with the letter WWW meaning: "Watches Wristlet Waterproof" and called the "Mark X" watches. Many manufacturers including Longines, Jaegre Le-Coutlre (JLC), Omega, Lemania, Eterna and the International Watch Company (IWC) supplied WWW Mark X watches which became available in 1944. These Mark X watches were supplied to the RAF, but again found wanting.

While dead reckoning and astro navigation were being practiced, other measures had been taken to drastically improve the standard of navigation. An airborne map painting radar system called ‘H2S’ were established and Allied aircraft could determine their position by triangulating their position with these beams. Still except for shorter raids over the channel, the range of these beams meant the air crews still relied on a combination of the radio, the stars and dead reckoning to determine position.

H2S proven to be a valuable navigation aid but also caused problems. The map painting radar screen produced magnetic fields, which disturbed the accuracy of the issued wrist watches.

Also as, the altitude of the sorties had steadily increased, causing a problem with low temperature, which also affected the precision of the watches. Cabin pressurization had been introduced towards the end of WW II and a sudden decrease of pressure caused by damage to the aircraft's fuselage, could force the crystal from a watch that still retained normal pressure inside its case. This resulted in a need for a watch that was not only resistant to vibration and the elements, but also to sudden changes in pressure.

Mark XI. To meet these new requirements new specification were issued for a Mark XI watch.

• A highly accurate movement with hack-device,
• A inner soft iron cage forming a shield to screen the movement against magnetic interference, the dial being an integral part of this soft iron cage,
• A stainless steel waterproof case,
• A crystal secured by a screwed ring to prevent it detaching from the case during sudden depressurization, and;
• A black dial with luminous markers and hands.

In 1949 Mark XI watches produced by JLC and IWC were issued to the RAF meeting these specifications. Mark XI watches were also purchased by the Royal Australian Air Force, Royal New Zealand Air Force, South African Air Force and the British Overseas Airways Corporation (now British Airways).

The RAF decommissioned the JLC Mark XI's by 1963, and the IWC built Mark XI's were decommissioned in 1981, after 32 years of service. The Mark XI represented a major development in the aviators watch.

IWC still manufactures its original Mark XI watch, with some improvements, the current version being the IWC Mark XVI. Following from its heritage as a military watch, it is resistant to magnetism, has a glass secured against drops in air pressure and is water resistant to 60 meters.

IWC Mark XVI

The case has a diameter of 39 mm, which is larger than the 36mm of the original Mark XI. The watch is available only in stainless steel, with a bracelet or black leather strap.