Cabin Heaters and Engine-Driven Blowers

A cabin heater supplies warm air to the cabin of an aircraft. Jet transports take hot compressed air from their engines and condition it in air conditioning packs, but light piston aircraft have no such supply. Most of them warm fresh ram air on the engine exhaust, and some larger piston and turboprop aircraft burn fuel in a separate combustion heater. Pressurised aircraft without a supply of clean bleed air, such as some piston-engined and turboprop types, compressed their cabin air with engine-driven blowers, or cabin superchargers, which could heat that air as well.

For pilots, one fact dominates the subject. Every method that heats air with combustion brings exhaust gas, and with it carbon monoxide, close to the air the occupants breathe. A cracked heater muff is the classic route by which carbon monoxide enters a light-aircraft cockpit, and the same duct is closed in the engine fire drill to keep smoke out. The blower systems of older aircraft, with their spill and choke valves, remain part of the EASA ATPL systems syllabus.

Ram air ventilation and heating

An unpressurised light aeroplane is ventilated by ram air. Forward-facing intakes, for example in the wing leading edges or the nose, capture outside air and duct it to adjustable vents. For heating, part of the incoming air is passed over the engine exhaust and warmed. Hot and cold air are mixed in proportions set by the pilot's cabin heat and cabin air controls, and the used air leaves through vents in the underside of the fuselage.

One rule governs the whole design: the ventilating air may take heat from the exhaust, but it must never come into contact with the exhaust gases themselves.

Exhaust shroud cabin heating

The exhaust shroud cabin heater, also called the heater muff, exhaust muff or exhaust heater shroud, is a close-fitting metal jacket around a section of the exhaust pipe or the muffler. Ram air flows through the space between the jacket and the pipe and is heated by conduction from the hot pipe wall. Together the jacket and the pipe form an exhaust heat exchanger: heat passes through the metal while the two streams stay apart. From the muff the warm air is ducted to a valve on the firewall, worked by the cabin heat control, and into the cabin.

The system is simple, light and uses heat that would otherwise be thrown away, which is why most single-engine training aeroplanes have it. The FAA's Aeronautical Information Manual (AIM) notes that most light-aircraft heaters work by air flowing over the exhaust manifold. The same heat source serves carburettor heat, whose air is also drawn from around the exhaust. That air bypasses the intake filter, which is why carburettor heat is used as little as possible on the ground (see carburettor icing).

Because the heater duct runs from the engine compartment through the firewall into the cabin, it is also a path for smoke and flames. The engine fire drill of a typical light single therefore closes the cabin heat and cabin air along with shutting off the fuel (see engine failure and engine fire).

The carbon monoxide hazard

The weakness of the heater muff is that only a thin metal wall separates the cabin air from the exhaust. Carbon monoxide (CO) comes from the incomplete combustion of carbon, and ATPL texts put its concentration at up to 9 per cent in piston-engine exhaust, against less than 1 per cent in jet exhaust. If the muffler or an exhaust pipe inside the shroud cracks, or a joint or seal leaks, exhaust gas mixes with the ventilating air and is blown straight into the cabin. According to the AIM, heater use while exhaust fumes escape through manifold cracks and seals causes several fatal and non-fatal accidents every year.

The gas is colourless, odourless and tasteless. It binds to haemoglobin far more readily than oxygen, producing an anaemic (FAA: hypemic) hypoxia that builds up insidiously and dulls the judgement needed to recognise it. Any smell comes from other exhaust products, so its absence proves nothing. For transport aeroplane certification, CS 25.831 and 14 CFR 25.831 treat a concentration above 1 part in 20,000 parts of air (50 ppm) as hazardous, and ATPL systems texts give the same figure as the limit for cabin air. Exposure is most likely in cold weather with the heater on.

A pilot who smells exhaust, or who develops a headache, drowsiness or dizziness while using the heater, should suspect carbon monoxide. The AIM's instruction is to shut off the heater immediately and open the air vents; training texts add stopping any smoking, using oxygen if it is carried and landing as soon as possible. Descending alone does not cure it, because the carbon monoxide stays bound to the haemoglobin, although a lower altitude removes the altitude hypoxia that adds to its effects. The aeroplane should not fly again until the exhaust has been inspected. The physiology and the detectors are covered in carbon monoxide and cabin air hazards.

Prevention rests on regular inspection of the exhaust system for cracks and on a carbon monoxide detector in the cabin. A chemical spot detector darkens as CO accumulates but warns only if the pilot looks at it; an electronic detector gives an aural or visual alarm at a set concentration.

Combustion heaters

Aircraft too large to be heated by an exhaust muff may use a combustion heater. It burns the aircraft's own fuel in a sealed combustion chamber that has nothing to do with the engines. A fan or blower supplies the combustion air, a solenoid valve controls the fuel and the combustion gases are vented overboard. The ventilating air for the cabin flows through a jacket around the chamber, which acts as the heat exchanger, and is warmed by the chamber walls. As with the muff, the two airstreams must be completely separate.

Outlet temperature is controlled by duct temperature sensors or thermal cycling switches that open and close the fuel valve. Because a failure could start a fire or put combustion gases into the cabin, a combustion heater must have:

• automatic fuel shut-off in the event of a malfunction, such as a flame-out or an overheat;
• adequate fire protection in case the combustion chamber loses its structural integrity;
• automatic shut-off if the outlet air becomes too hot.

On many installations the overheat switch shuts the heater down and can only be reset on the ground by maintenance. Combustion heaters are found in older light twins, such as those fitted with Janitrol heaters, and in some turboprops. They are subject to airworthiness directives and regular inspection, because a cracked heat exchanger admits exhaust and carbon monoxide to the cabin. On smaller aircraft without bleed air, the same device can also supply hot air for airframe de-icing.

Exhaust heat exchangers on turboprops

Some turboprops apply the heater muff principle on a larger scale. Exhaust gas can be diverted through a heat exchanger in which outside air, flowing through tubes on its way to the supply ducts, is heated by contact with the hot tubes while remaining separate from the exhaust. A thermostatically controlled flap in the ducting between the exhaust unit and the heat exchanger regulates the temperature, and the heated air can serve the cabin or the ice protection of the leading edges.

Engine-driven cabin superchargers

A pressurised cabin needs a continuous supply of compressed air. Modern turbine aircraft take it as bleed air from the engine compressors (see bleed air and pneumatic systems). Where clean bleed air is not available, as on piston-engined and some turboprop aircraft, or on turbojets whose bleed air could be contaminated, the air comes from a cabin supercharger, also called an engine-driven blower.

The blower is a separate compressor of centrifugal or positive-displacement (Roots) type. It is driven through the engine accessory gearbox, or by a turbo-compressor powered by bleed air, which compresses clean outside air instead of passing the engine's own air to the cabin. It delivers the mass flow of air that ventilates and pressurises the cabin (see pressurisation principles and control). Light pressurised twins with turbocharged piston engines take their cabin air from the turbocharger compressors instead (see supercharging and turbocharging).

Spill valves and mass flow control

A blower geared directly to the engine delivers a flow that rises with engine rpm and air density. At low altitude and high rpm it supplies far more air than the cabin needs. A mass flow controller then signals one or more spill valves, which vent the excess to atmosphere before it can overpressurise the supply ducts.

Spilling is wasteful: the engine has done the work of compressing air that is then thrown away. Variable-speed blower drives, which match the delivery to the demand, avoid it and are preferred. Bleed-air systems regulate in a different way. Their mass flow controller is a calibrated variable-orifice valve that holds a constant flow to the air conditioning pack whatever the pressure upstream, a regulating valve rather than a relief.

Choke valve heating

A blower can heat the air it delivers without any separate heater. A choke valve in the blower outlet duct is progressively closed to restrict the flow. The blower then works harder against the restriction, and the air leaves it at a higher pressure and temperature: the heat of compression, the same effect that makes compressor delivery air hot in a gas turbine (see gas turbine principles). Opening the choke valve lowers the temperature again. Hot air from the choked supply is then mixed with cold air in varying proportions, automatically or manually, to give a comfortable delivery temperature.

Why bleed air took over

Bleed air displaced the engine-driven blower on turbine aircraft for three reasons. It needs no additional accessory drive on the engine. It uses air that the engine compressor has already compressed. And by tapping the compressor at a suitable stage, the system can match the flow to the demand without spilling the surplus. Its costs are a small loss of thrust and a rise in fuel consumption while air is being bled.

The bleed air of a jet transport also has one advantage over every combustion heater: the pneumatic system normally has no contact with combustion gases, so carbon monoxide is not the concern it is in a light aeroplane. Contamination can still occur, from engine oil leaking into the compressor air in a fume event, which is covered with the other cabin air hazards.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.