the pitot heads on the nose compartment. The conventional airspeed scale is calibrated in knots and the Mach scale is calibrated in percentage of Mach and connected to an aneroid that moves the scale to compensate for changes in pressure altitude. Air conditioning and pressurization Engine bleed air is admitted through a flow control valve to a heat exchanger. The cabin temperature is controlled by regulating the temperature of the pressure bleed air that is routed through the cabin. The bleed air is cooled in the heat exchanger by ram air entering the dorsal fin inlet and passing through the heat exchanger. The amount of bleed air cooling at the heat exchanger can be controlled by the hot air bypass valve (H-valve). The H-valve position can be adjusted by the crew to increase or reduce the amount of bleed air cooling at the heat exchanger. A refrigeration-type cooling system is used for cooling and dehumidification while the aircraft is on the ground or operating at altitudes below 18 000 feet. The refrigeration system consists of a compressor, receiver dehydrator and evaporative cooler located above the baggage compartment. The Learjet 25 cabin is pressurized to enable high altitude operations without the use of supplemental oxygen. The cabin pressure is provided by the conditioned air entering the cabin through the distribution ducts and controlled by modulating the amount of air exhausted from the cabin. During ground operations, a solenoid limits the pressure differential to 0.25 psi in order to ensure normal functioning of the door and emergency exit. The pressure differential at final cruising altitude will be maintained at 8.7 psi difference between pressure altitude and cabin altitude. A rate controller enables the crew to select the rate pressurization of the cabin within preset limits. The normal pressure relief valve will open at 8.9 psi differential pressure and the safety outflow valve will open at 9.2 psi differential pressure which is the maximum allowable differential pressure. Oxygen is contained in a pressurized bottle located in the dorsal fin of the aircraft. The use of oxygen is only required for emergency in case of depressurization of the cabin or contamination of the cabin air. Oxygen is always available for the crew and can be made available to the passengers manually or automatically. The oxygen storage cylinder has a capacity of 38 cubic feet and is stored at 1800 psi. An oxygen rupture disc