Takeoff and Landing Gear Warnings
Areas of Fire Protection and Fire/Overheat Detection
Declaring an Emergency and Hijacking
All aircraft have a warning system to alert the crew that the landing gear is not down and locked when the aircraft is in a landing configuration. This warning system consists of a horn and red gear lights. It will start when at least one throttle is retarded and the landing gear is not down and locked. See Figure 12-1.
An intermittent warning horn sounds when the aircraft is on the ground, the throttles are advanced, and the aircraft is not in a takeoff configuration. Conditions which will trigger the horn are: flaps not set in the takeoff position, trim not in the takeoff range, or ground spoilers not retracted. This horn is not dependent on thrust but on the position of one or more of the throttle levers. When the throttles are pushed forward, a switch closes that arms the takeoff warning system.
Figure 12-1
Areas typically protected by a fire or smoke detection system include the engines and engine nacelles, the APU, the wheel wells, cargo compartments and lavatories.
Normally, only areas of the aircraft that are inaccessible have a built-in fire protection system. These areas include the engine and engine nacelles, and the APU. While the wheel wells are inaccessible in flight, they are not normally equipped with fire extinguishers because lowering the landing gear will usually take care of any fire or overheat condition.
Usually the fire warnings in the cockpit consist of a light for each area protected by a detection system and an alarm bell that sounds when a fire is detected in any protected system. The bell can be silenced but will ring again if a fire is detected in any other area.
The thermocouple-type detection system triggers a fire alarm based on the rate of temperature rise rather than detecting a rise above a preset value. Thermocouples are placed in areas where a fire is most likely to occur and a reference thermocouple is placed in an area relatively well-protected from the initial flame. When a fire occurs, one or more of the active thermocouples will heat up much faster than the reference. The difference in temperature will produce a current in the thermocouple loop and the fire alarm will be activated. If the entire system, including the reference thermocouple is heated evenly, no alarm will sound regardless of the maximum temperature reached.
A continuous-loop fire detection system has a loop with two wires separated by a ceramic material that acts as an insulator when cool, and an electrical conductor when hot. If any part of the loop gets hot enough, the ceramic allows a current to flow between the two wires and the fire alarm is initiated. Kinks, dents or crushed sensors in a continuous loop system can generate false alarms by causing a short circuit in the internal wiring.
A two-wire thermal switch system has individual sensors wired in parallel in the fire detection circuit. When one or more of these sensors detects an overheat condition, the alarm is activated. If one sensor has a short or open circuit, fire detection is lost in that sensor's area, but the system will continue to provide fire detection from its other sensors.
Smoke detectors sample the air from cargo compartments for the presence of smoke. The two primary systems are the photoelectric and the visual detection types.
A photoelectric smoke detector has a photoelectric cell, a beacon lamp, a test lamp and a light trap. These detectors measure the light transmissibility of air. If there is any smoke in the detector, some light from the beacon will be refracted into the photoelectric cell and a smoke alarm will be initiated.
The visual smoke detector has an optical indicator mounted on the flight engineer's panel. Cargo compartment air circulates through the indicator. A light beam constantly illuminates the inside of the indicator. If smoke is present, it will be visible. If the air sample contains no smoke, the indicator window will be dark. An indicator light on the detector shows that the light inside the detector is on.
When an engine fire alarm rings, a red light illuminates in the applicable engine fire handle. The appropriate crew action is to pull the fire handle. This will do several things to isolate the engine fire and deny it fuel. Pulling the fire handle:
Pulling the fire handle uncovers the fire extinguisher switch beneath the handle. When this switch is depressed, the contents of a fire extinguishing bottle discharges into the engine. Carbon dioxide is often the extinguishing agent. Nitrogen is often added to make the bottles less susceptible to thermal discharge.
In most aircraft, a second bottle can be discharged to an engine by using a bottle transfer switch. After the first bottle is discharged, the transfer switch must be moved to the alternate position. When the fire switch is pressed a second time, the contents of the other bottle will be discharged into the engine. Because it takes time for sensor-responder type fire detectors to cool enough for the fire light to go out, thirty seconds should be allowed after discharging a fire bottle before using a second bottle on the same engine.
Part of the flight engineer's preflight inspection is to check the status of the fire extinguisher system indicator discs. Each fire bottle has a safety discharge port that allows the extinguishing agent to vent overboard if bottle pressure becomes excessive (usually due to overheating). Each of these ports is covered by a red plastic disc. If a red disc is ruptured or missing, it means there has been a thermal discharge from the one fire extinguishing bottle associated with that port.
Some extinguishing systems have an externally mounted yellow plastic disc to indicate an intentional discharge of one or more of the bottles in the system. There is only one yellow disc for each system. The yellow disk is always visible, unless there has been an intentional discharge of the bottle.
Each extinguisher bottle has a pressure gauge to determine if an adequate charge is available from that bottle. Some systems also have a low pressure light for each bottle. This light will warn the flight crew of the loss of bottle pressure and can be used to confirm bottle discharge in the event of a fire.
There are four types of fire extinguishers that may be available for use in aircraft. It is important that flight crewmembers know the proper type of extinguisher to use on each type of fire and the markings that identify the extinguisher. See Figure 12-2.
Type A extinguishers are suitable for use on ordinary combustibles such as paper or wood. A type A extinguisher is identified by a triangle with the letter "A." The extinguishing agent is often water.
Type B extinguishers are suitable for use on flammable liquids. The extinguisher bottle is identified by a square and the letter "B."
Type C extinguishers are suitable for use on electrical fires. Type "C" extinguishers are identified by a circle and the letter "C." Many extinguishing agents are usable for either class B or C fires. One of the most common agents is carbon dioxide. Not only is it an effective fire-fighting agent, but it is the safest from the standpoint of toxicity and corrosion.
Type D extinguishers are used on fires involving combustible metals such as magnesium. They are identified by a star and the letter "D." This is usually a dry powder type agent and is used on brake fires. If a type A extinguisher is used on a class D fire it could actually cause the fire to intensify.
Figure 12-2. Portable fire extinguisher identification
An emergency oxygen system is installed in all airline-type aircraft in case there is a loss of cabin pressurization. Without supplementary oxygen at high cabin altitudes, a person experiences many adverse physical and psychological symptoms. Hypoxia is a reversible condition, by using oxygen or descending to a lower altitude. When the lack of oxygen results in permanent physical damage, the condition is called anoxia.
The oxygen used in aircraft must be free of contaminants and contain no more than 2 milliliters of water per liter of gas. This dryness is essential to ensure that water will not condense inside the valves and lines, then freeze and cause a system malfunction. Aircraft oxygen bottles are painted green and labeled "AVIATOR'S BREATHING OXYGEN."
Oxygen systems are usually equipped with a thermal relief valve that allows oxygen to vent overboard if the system becomes over-pressurized. If this occurs, a green blowout disk on the outer skin of the aircraft will be ruptured or missing.
Great care must be taken when working with an oxygen system because combustible materials ignite more readily and burn with greater intensity in an oxygen-rich environment. When petroleum products are exposed to pure oxygen, spontaneous fires or explosions can result.
The typical crew oxygen system uses a diluter-demand type regulator. When the regulator is in the "normal" position, cabin air mixes with oxygen. A greater percentage of oxygen is delivered through the regulator as cabin altitude increases. Oxygen duration will vary with altitude in the diluter-demand mode. The user will get almost no supplementary oxygen at sea level. As the cabin altitude increase, so does the percentage of oxygen flowing to the user until approximately 34,000 feet, where he/she gets 100% oxygen. See Figure 12-3.
When the diluter-demand regulator is selected to the 100% position, pure oxygen is supplied through the regulator at all altitudes. Oxygen duration will be the same at all altitudes in the 100% mode.
Diluter-demand would be used when there is a loss of cabin pressure because it stretches the oxygen supply. The 100% mode would be used to prevent hypoxia and hyperventilation. It would also be used if there was smoke in the cabin, to avoid contaminating the oxygen with smoke.
Many regulators also have an "emergency" switch that controls the flow through the regulator. When "emergency" is selected, 100% oxygen flows continuously through the regulator under positive pressure. This mode is used at very high cabin altitudes.
Almost all crew oxygen systems use an oronasal type oxygen mask that covers only the nose and mouth. Usually smoke goggles are available if needed. During preflight, each crewmember should check his/her mask for proper operation and to make sure that it forms an airtight seal on the face. The masks may be cleaned before the test with a mild antiseptic solution.
Figure 12-3. Oxygen regulator
The gaseous passenger oxygen system operates only in the continuous flow mode. When activated, the oxygen system pressurizes, and oxygen masks drop from the passenger service units above the passenger's seats. Oxygen does not flow through a mask until a passenger pulls it down to his/her face. When he/she does, a valve opens in the line to that mask and 100% oxygen flows to it. A calibrated orifice controls the amount of oxygen delivered to the mask. The oxygen fills a bag hanging below one type of mask, and when the passenger inhales he/she draws the oxygen rich mixture from the bag until it is empty. If he/she continues to inhale, he/she breathes cabin air mixed with the small amount of oxygen flowing into the bag at that moment. When he/she stops inhaling, the bag is refilled by the constant flow of oxygen.
Some aircraft have chemical oxygen generators instead of a gaseous passenger system. In this type of system, individual oxygen generating units are located in the passenger seat backs or overhead compartment. When activated, each unit produces oxygen through a chemical reaction taking place in that unit. This type of system eliminates the need for oxygen lines for the passenger system which reduces the fire hazard. It is also much easier to maintain.
A disadvantage of the chemical system is that once it is activated, it cannot be turned off and will continue to generate oxygen until its chemical supplies are expended. The chemical reaction also generates a great deal of heat and there is some danger of a passenger burning him/herself if the unit is touched. The units are marked with a heat sensitive paint mark that changes color from white to black when the unit is used.
If an emergency occurs in flight, ATC should be notified as soon as possible. This should be done over the ATC radio frequency in use at the time. The frequencies 121.5 MHz VHF and 243.0 MHz UHF are set aside for emergency use but should be used only when no ATC frequency has been assigned and the crew with the emergency is forced to broadcast "in the blind."
If a pilot declares a "minimum fuel" situation to ATC, this does not constitute an emergency. It does signal ATC that any undue delay may result in declaration of a low fuel emergency.
There are a couple of procedures that will alert the authorities if an aircraft is being forced to a new destination. The transponder should be set to 7500. The ATC controller will ask you to confirm the 7500 code but will not indicate that he/she believes a hijacking is in progress unless you broadcast that information on the radio. If possible, you should maintain a true airspeed of less than 400 knots and an altitude between 10,000 feet and 25,000 feet.
If a two-way communications radio failure occurs in flight, the transponder should be set to code 7600. Airport control towers will signal instructions to "no radio" aircraft with a light gun. See Figure 12-4.
Figure 12-4