Most pressurisation systems have three basic cockpit indicators cabin altitude, cabin rate of climb and the pressure differential indicator. The cabin altitude gauge measures the actual cabin altitude.
Cabin Altitude Gauge Figure 32
The cabin rate of climb indicator tells the pilot the rate that the cabin is either climbing or descending. (I.e. the rate at which the cabin loses or gains pressure) A typical maximum climb rate is 500ft per minute and the maximum descent rate is 300ft per minute. The control can be automatic or manual depending on aircraft type.
Cabin Rate of Climb Figure 33
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JAR 66 CATEGORY B MODULE 11.04 AIR CONDITIONING AND CABIN PRESSURISATION
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The differential pressure gauge (Figure 34) reads the difference between the cabin and the outside air pressures. This differential pressure is normally controlled and maintained to a structural limitation around 7psid. This depends on the aircraft type and the operating ceiling of the aircraft. The differential pressure gauge may be combined with the cabin altitude (Figure 35).
Differential Pressure Gauge Dual Gauge Figure 34 Figure 35 4.12 SAFETY AND WARNING DEVICES
To ground test the pressurisation system with the engines running, at least three men are required inside the aircraft for safety reasons.
Both air conditioning and pressurisation systems use safety and warning devices to protect the aircraft from possible catastrophic failures. Some of the protection devices may be inhibited in certain stages of flight; landing or take off where the extra distractions caused by such warnings may be too much for the crews to deal with safely.
With the air conditioning system the main concerns are with overheating of the air conditioning packs and extraction and ventilation fans, as well as hot air leaks from ducting which could damage surrounding structure or components.
4.12.1 Overheating
Most packs systems are protected from overheating by a thermal switch downstream of the pack outlet. If the outlet temperature reaches a pre determined figure the switch will operate causing the pack valves to shut, preventing air from getting to the packs, as well as sending a warning signal to the cockpit central warning panel with associated caution/warning lights and aural chimes and to illuminate a fault light on the pack selector switch.
0 1 2 3 4 5 6 7 8 9 10 DIFF PX PSI
Issue 1 - 20 March 2001 Page 4-37 Once the system has cooled down sufficiently the crew may have an option to reselect the overheated system. The overheat may have been caused by a fault in the automatic temperature control system in which case the pilot may be able to control the system manually via a manual selector switch on the cockpit controller.
Extraction or ventilation fans will be protected in much the same way. An overheat will signal the central warning panel with associated caution/warning lights and aural chimes. The fan may be isolated automatically or manually. Once the fan has cooled down it may be possible to re-select if required. Fans may also be protected from over or under speeding, which will also have an effect on the system temperatures. Speed sensors on the fan will indicate a fault when over or under speed limits are reached and a warning signal is sent to the cockpit central warning panel with associated caution/warning lights and aural chimes.
4.12.2 Duct Hot Air Leakage
Any ducting that includes joints is liable to leak under abnormal conditions. A duct protection system will include fire-wire elements around the hot zones such as engine air bleeds, air conditioning packs and auxiliary power units if fitted.
The sensing elements will be the thermistor type. As the temperature around the wire increases the resistance decreases until an electrical circuit is made. When the circuit is made a warning signal is sent to the cockpit central warning panel with associated caution/warning lights and aural chimes. The leaking duct may be isolated automatically or may require the pilot to take action to close off the air valves. The faulty system will then remain out of use.
4.12.3 Excess Cabin Altitude
If the cabin altitude was allowed to increase unchecked the crew and passengers could unknowingly suffer the effects of hypoxia. This dangerous condition is obviously undesirable especially for the aircrew. Most aircraft give a warning on the CWP with associated audio and visual warnings when the cabin altitude reaches 10000`.
4.12.4 Smoke Detection
Smoke detectors may be fitted within the cabin; avionics bay and cargo areas to monitor systems, which if become faulty may generate smoke on overheating, or are may be liable to catch fire. These detectors will send a signal to the CWP with associated lights and audio warnings. They may also automatically switch on extractor fans, which will remove the smoke overboard and away form the cabin and cockpit areas. In this event, the pilot may have a switch or control lever to operate a valve to isolate the cockpit air conditioning ducting from the rest of the aircraft to prevent any smoke from getting to the cockpit.
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B1 Mod 11.07 Issue No 0 30 - Jan 2003 Page 7-1