SEPECAT JAGUAR KELVIN & HUGHES Mk 3(P) RATE OF CLIMB INDICATOR

There are usually six primary instruments on an aircraft's main flying panel which enable the pilot to determine his aircraft’s position in the sky.

They consist of the Airspeed Indicator, Attitude Indicator, Heading Indicator and Turn Coordinator. The final instrument in the aircraft's ‘six-pack’ is the Vertical Speed or Climb Indicator.

Vertical airspeed is the rate at which an airplane ascends or descends. Differing from ground speed, it displays the aircraft’s rate of climb by tracking the airplane’s vertical airspeed, and the rate of descent, or sink rate, which indicates how quickly the airplane is descending.

Displayed as feet per minute, or FPM, like many of the six-pack’ instruments, the Rate of Climb Indicator is connected to the aircraft’s pitot-static system which is often seen as an ‘L’ shaped tube protruding from the airframe or wing and measures air velocity at right angles to the direction of airflow.

Through a series of rods, gears and a flexible diaphragm, the airflow entering the port informs the vertical speed indicator display, which is traditionally a needle dial. Modern cockpits might feature a digital Vertical Speed Indicator. 

Displaying the rate at which the airplane is moving along the vertical axis, the larger the pressure differential between the casing of the mechanism and the diaphragm, the more dramatic the movement of the needle.

When an airplane is in level flight, an opening within the diaphragm called the ‘ calibrated leak’ bleeds off the air within. This then resets the Vertical Speed Indicator to zero until the aircraft climbs or descends again.

In powered flight, the pilot makes frequent use of the instrument to ascertain that level flight is being maintained, especially during turning manoeuvres.

Pilots of older aircraft might notice that when the needle on the vertical speed indicator starts to move, it might not give precise information about how quickly the airplane is rising or dropping. This is what is known as a “trend.” However, once the diaphragm, mechanism, and data relay are caught up with what is taking place in air pressure differentials, the Vertical Speed Indicator quickly stabilises, and the actual rate becomes available.

The gap between seeing the trend and knowing the actual rate can last anywhere between six to nine-seconds  Small changes which are made slowly usually show up more quickly on a traditional Vertical Speed Indicator, whilst sudden shifts in pitch can result in lags even longer than nine-seconds.  

In some situations and in some aircraft, this can cost precious decision-making time and cause a pilot to assume that they have failed to pitch the airplane correctly, and to then overcorrect. Pilots who are struggling with a lagging Vertical Speed Indicator are said to be “chasing the needle.”

More modern aircraft feature instantaneous Vertical Speed Indicators which incorporate small accelerator actuated pumps in tandem with air flow sent from the pitot tube to provide the pilot with instant vertical speed information rather than waiting for the more traditional pitot-static system to catch up.

This Kelvin Hughes Mk. 3(P) Vertical Speed Indicator is of the traditional design and was installed in the early Sepecat Jaguars which had a vertical climb ability of reaching over 30,000 ft in less than 1 min 30 sec.

With its undamaged bakelite case and clear, scratch-free glass face, this Kelvin Hughes Mk. 3(P) Vertical Speed Indicator, presents an amazing opportunity to secure a rare piece of RAF Cold War/Gulf War aviation history from an operational Sepecat Jaguar.   

This English Sepecat Jaguar Instrument comes complete with detailed Scale Model, Mango Wood Stand & Plaque plus Printed Fact Sheet featuring photo of instrument in aircraft cockpit.

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