B-25 MITCHELL BOMBER, FREE AIR TEMP GAUGE

Installed at the bottom right of the B-25 pilot’s instrument panel just to the left of the aircraft’s Fuel Gauges, the Free Air or Outside Air Temperature (OAT) gauge alerted the pilot to the outside air temperature.

Knowing the outside air temperature does not on the face of it, seem all that important, but to a pilot it provides the information necessary to make true airspeed and altitude density calculations essential to flight planning and aircraft fuel consumption.

The OAT gauge also serves as a critical early warning to potential icing conditions. Changes in altitude and passing through cloud or precipitation can cause the rapid formation of ice on the wing surfaces which can have dramatic effect on flight surfaces and aircraft stability.

At outside temperatures of between zero and minus 10 C the pilot is likely to encounter ‘clear icing’ where a glossy, thin and transparent layer of ice forms slowly over the wing surfaces reducing clean airflow over the wing increasing drag and reducing lift.

As OAT falls further, Rime ice can form which is a rougher and faster forming ice, trapping air pockets which reduce air flow and further increases drag. At higher altitudes in extreme conditions, rough Mixed ice can also form which will dangerously impact the aircraft’s aerodynamics.

As ice formation is difficult for the pilot to see from the cockpit, its often called the silent creeper in aviation circles and without keeping an eye on the OAT, the first thing a pilot might experience is significant loss of wing lift resulting in a sudden stall which may be difficult to recover from.

Like its bigger contemporary the B-17, the Mitchell also had a de-icing system installed along the leading wing and tail fin edges of the aircraft.

The B-25 cruised at a height of over 24,000ft where temperatures often dropped to well below freezing, requiring air crews to wear thick, electrically heated flying suits, gloves and helmets.

Ice build up on wing and flight surfaces were an ever-present problem and required constant monitoring by the copilot and engineer. B-25’s featured Rubber De-Icing Boots which were overlaid along the aircraft’s leading wing and tailplane edges.

Small bladders were distributed under the Boots which enabled the crew to inflate and deflate the boots to break off any accumulated wing ice through a series of small tubes beneath the B-25’s metal skin.

The upper illustration shows a leading edge boot as it is held against the leading edge contour (normal in-flight condition). The lower illustration shows a leading edge boot in an inflated condition; the boot is typically in this position only for a short time to remove an accumulation of ice.

Manufactured by Weston Electric Inst Corp this 115412 T-52H, this Free Air Temperature Gauge comes complete with Manufacture’s plate and would look fantastic with its highly detailed 1/72 scale B-25 Mitchell bomber model perched atop its 100yr old Mango Wood stand. What a great original one-of-a-kind gift for any aviation enthusiast or budding, active or retired pilot!

This B-25 Mitchell bomber instruments come complete with detailed Scale Model, Mango Wood Stand & Plaque plus Printed Fact Sheet featuring photo of instrument in aircraft cockpit.

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