Environmental Control System (ECS)

The Environmental Control System of an airliner provides air supply, thermal control and cabin pressurization for the passengers and crew. Avionics cooling, smoke detection, and fire suppressionare also commonly considered part of the Environmental Control System.

On most jetliners, air is supplied to the ECS by being "bled" from a compressor stage of each gas turbine engine, upstream of the combustor. The temperature and pressure of this "bleed air" varies widely depending upon which compressor stage and the RPM of the engine.

A "Manifold Pressure Regulating Shut-Off Valve" (MPRSOV) restricts the flow as necessary to maintain the desired pressure for downstream systems. This flow restriction results in efficiency losses. To reduce the amount of restriction required, and thereby increase efficiency, air is commonly drawn from two bleed ports (3 on the Boeing 777).

When the engine is at low thrust, the air is drawn from the "High Pressure Bleed Port." As thrust is increased, the pressure from this port rises until "crossover," where the "High Pressure Shut-Off Valve" (HPSOV) closes and air is thereafter drawn from the "Low Pressure Bleed Port."

To achieve the desired temperature, the bleed-air is passed through a heat exchanger called a "pre-cooler." Air from the jet engine fan is blown across the pre-cooler, which is located in the engine strut. A "Fan Air Modulating Valve" (FAMV) varies the cooling airflow, and thereby controls the final air temperature of the bleed air.

The Cold Air Unit, or "Airconditioning pack" is usually an air cycle machine (ACM) cooling device. Some aircraft, including early 707 jetliners, used vapor-compression refrigeration like that used in home air conditioners.

On most jetliners, the A/C packs are located in the "Wing to Body Fairing" between the two wings beneath the fuselage. On some jetliners (Douglas Aircraft DC-9 Series) the A/C Packs are located in the tail. The A/C Packs on the McDonnell Douglas DC-10/MD-11 and LockheedL-1011 are located in the front of the aircraft beneath the flight deck. Nearly all jetliners have two packs, although larger aircraft such as the Boeing 747, Lockheed L-1011, and McDonnell-Douglas DC-10/MD-11 have three.

The quantity of bleed air flowing to the A/C Pack is regulated by the "Flow Control Valve" (FCV). One FCV is installed for each pack. A normally closed "isolation valve" prevents air from the left bleed system from reaching the right pack (and v.v.), although this valve may be opened in the event of loss of one bleed system.

The A/C Pack exhaust air is ducted into the pressurized fuselage, where it is mixed with filtered air from the recirculation fans, and fed into the "mix manifold". On nearly all modern jetliners, the airflow is approximately 50% "outside air" and 50% "filtered air."

Modern jetliners use "High Efficiency Particulate Arresting" HEPA filters, which trap >99% of all bacteria and clustered viruses.

Airflow into the fuselage is approximately constant, and pressure is maintained by varying the opening of the "Out Flow Valve" (OFV). Most modern jetliners have a single OFV located near the bottom aft end of the fuselage, although some larger aircraft like the 747 and 777 have two.

In the event the OFV should fail closed, at least two Positive Pressure Relief Valves (PPRV) and at least one Negative Pressure Relief Valve (NPRV) are provided to protect the fuselage from over- and under- pressurization.

The atmosphere at typical jetliner cruising altitudes is generally very dry and cold, and the outside air pumped into the cabin on a long flight typically has a relative humidity around 10%. The fact that cabin pressure is generally lower than the pressure at ground level does not of itself contribute to the dryness.

AIRCRAFT MASS AND BALANCE

AIRCRAFT MASS AND BALANCE

The main purposes, of monitoring the mass and balance of aircraft, are to maintain safety and to achieve efficiency in flight. The position of loads such as passengers, fuel, cargo and equipment will alter the position of the Centre of Gravity (CG) of the aircraft.

Incorrect loading will affect the aircraft rate of climb, manoeuvrability, ceiling, speed and fuel consumption. If the CG were too far forward, it would result in a nose-heavy condition, which could be potentially dangerous on take-off and landing. If the CG is too far aft, the tail-heavy condition will increase the tendency of the aircraft to stall and make landing more difficult.

Stability of the aircraft will also be affected with the CG outside the normal operational limits. Provided the CG lies within specified limits, the aircraft should be safe to fly. The unit of measurement for mass and balance are normally dictated by the aircraft manufacturer and can be either Metric or Imperial terms. Specific definitions for mass and balance ensure they are correctly interpreted.

• Datum: The datum is an imaginary vertical plane from which horizontal measurements are taken. The locations of items such as baggage compartments, fuel tanks, seats and engines are relevant to the datum. There is no fixed rule for the location of the datum. The manufacturer will normally specify the nose of the aircraft, but it could be at the front main bulkhead or even forward of the aircraft nose

• Arm: The horizontal distance from an item or piece of equipment to the datum. The arm's distance is usually measured in inches (or millimetres) and may be preceded by a plus (+) or a minus (-) sign. The plus sign indicates that the distance is aft of the datum and the minus sign indicates distance is forward of the datum

• Moment: The product of a force multiplied by the distance about which the force acts. In the case of mass and balance, the force is the mass (kg/lb) and the distance is the arm (m/in). Therefore, a mass of 40 kilograms, at 3 metres aft of the datum will have a moment of 40 x 3 = 120 kg/m. It is important to consider whether a value is positive (+ve) or negative (-ve) when moments are calculated and the following conventions are used:

Distances horizontal: aft of the datum (+), forward of the datum (-).
Weight: added (+), removed (-).

• Centre of Gravity (CG): This is the point about which all of the mass of the aircraft or object is concentrated. An aircraft could be suspended from this point and it would not adopt a nose-down nor a tail-down attitude.

• Centre of Gravity Balance Limits: For normal operation of the aircraft, the CG should be between the Forward and Aft limits as specified by the manufacturer. If the CG is outside these limits, the aircraft performance will be affected and the aircraft may be unsafe.

• Dry Operating Mass: The total mass of the aeroplane, ready for a specific type of operation, excluding all usable fuel and traffic load. This mass includes crew and crew baggage, catering and removable passenger service equipment, potable water and lavatory chemicals.

• Maximum Zero Fuel Mass: The maximum permissible mass of an aircraft with no usable fuel.  Fuel contained in certain tanks must be included if this is explicitly mentioned in the aircraft’s Flight Manual limitations.

• Maximum Structural Take-Off Mass (MTOM): The maximum permissible total aeroplane mass at the start of the take-off run.

• Maximum Structural Landing Mass: The maximum permissible total aeroplane mass upon landing under normal circumstances.

• Traffic Load: This includes the total mass of passengers, baggage and cargo, including any non-revenue load.


Mass and Balance Documentation

The Mass and Balance documentation used by an operator must include certain basic information, which is listed below. Subject to the approval of the authority, some of this information may be omitted.

A. Aeroplane registration and type
B. Flight identification number and date
C. Identity of the commander
D. Identity of the person who prepared the document
E. Dry operating mass and the corresponding CG of the aeroplane
F. Mass of the fuel at take-off and the mass of trip fuel
G. Mass of consumables other than fuel
H. Load components that include passengers, baggage, freight and ballast
I. Take-off Mass, Landing Mass and Zero Fuel mass.
J. The load distribution
K. Aeroplane CG positions
L. Limiting mass and CG values


FREQUENCY OF WEIGHING

Aircraft must be weighed before entering service, to determine the individual mass and CG position. This should be done once all manufacturing processes have been completed. The aircraft must also be re-weighed within four years from the date of manufacture, if individual mass is used, or within nine years from the date of manufacture, if fleet masses are used.

The mass and CG position of an aircraft must be periodically re-established. The maximum interval between one aircraft weigh and the next, must be defined by the operator, but not exceed the four/nine year limits

15.7 CALCULATION OF MASS AND CG OF ANY SYSTEM

The position of the CG of any system (refer to Fig. 1) may be found using the following process:

• Total Mass is calculated, by adding the mass of each load (plus the mass of the beam)

• The moment of each load is calculated, by multiplying the mass by the arm (distance from the reference datum)

• ALL the moments are added together, to provide the Total Moment

• Total Moment is divided by the Total Mass to give CG position.

Aviation Hydraulic Oil

Aircraft hydraulic fluids fall under various specifications:

Common petroleum-based:

• Mil-H-5606: Mineral base, flammable, fairly low flashpoint, usable from −65 °F (−54 °C) to 275 °F (135 °C), red color
• Mil-H-83282: Synthetic hydrocarbon base, higher flashpoint, self-extinguishing, backward compatible to -5606, red color, rated to −40 °F (−40 °C) degrees.
• Mil-H-87257: A development of -83282 fluid to improve its low temperature viscosity.

Phosphate-ester based:

• BMS 3-11: Skydrol 500B-4, Skydrol LD-4, Skydrol 5 and Exxon HyJetIV-A plus – Typically light purple, not compatible with petroleum-based fluids, will not support combustion.

Radial Engines

Radial Engines

Nose Section

In general, it is either tapered or round in order to

place the metal under tension or compression instead

of shear stresses.

A tapered nose section is

used quite frequently on direct-drive, low-powered

engines, because extra space is not required to house

the propeller reduction gear.

The nose section on engines which develop from

1,000 to 2,500 hp. is usually rounded and sometimes

ribbed to get as much strength as possible.

Aluminum alloy is the most widely used material

because of its adaptability to forging processes and

its vibration-absorbing characteristics.

Since the nose section transmits many varied

forces to the main or power section, it must be

properly secured to transmit the loads efficiently.

It also must have intimate contact to give rapid and

uniform heat conduction, and be oiltight to prevent

leakage. This is usually accomplished by an offset

or ground joint, secured by studs or capscrews.

On some of the larger engines, a small chamber

is located on the bottom of the nose section to collect

the oil. This is called the nose section oil sump.

Power Section

On engines equipped with a two-piece master rod

and a solid-type crankshaft, the main or power

crankcase section may be solid, usually of aluminum

alloy.

This portion of the engine is often called the

power section, because it is here that the reciprocating

motion of the piston is converted to rotary

motion of the crankshaft.

Because of the tremendous loads and forces from

the crankshaft assembly and the tendency of the

cylinders to pull the crankcase apart, especially in

extreme conditions when a high-powered engine is

detonated, the main crankcase section must be very

well designed and constructed.

The machined surfaces on which the cylinders

are mounted are called cylinder pads. They are

provided with a suitable means of retaining or

fastening the cylinders to the crankcase.

The inner portion of the cylinder pads are sometimes

chamfered or tapered to permit the installation

of a large rubber O-ring around the cylinder

skirt, which effectively seals the joint between the

cylinder and the crankcase pads against oil leakage.

Diffuser Section

The diffuser or supercharger section generally is

cast of aluminum alloy, although, in a few cases,

the lighter magnesium alloy is used.

Because of the elongation and contraction of the

cylinders, the intake pipes which carry the mixture

from the diffuser chamber through the intake valve

ports are arranged to provide a slip joint which

must be leakproof. The atmospheric pressure on

the outside of the case of an unsupercharged engine

will be higher than on the inside, especially when

the engine is operating at idling speed. If the

engine is equipped with a supercharger and operated

at full throttle, the pressure will be considerably

higher on the inside than on the outside of the case.

If the slip joint connection has a slight leakage,

the engine may idle fast due to a slight leaning of

the mixture. If the leak is quite large, it may not

idle at all. At open throttle, a small leak probably

would not be noticeable in operation of the engine,

but the slight leaning of the fuel/air mixture might

cause detonation or damage to the valves and valve

seats.

Accessory Section

The accessory (rear) section usually is of cast

construction, and the material may be either aluminum

alloy, which is used most widely, or magnesium,

which has been used to some extent.

the

increased demands for electric current on large

aircraft and the requirements of higher starting

torque on powerful engines have resulted in an

increase in the size of starters and generators.

This means that a greater number of mounting bolts

must be provided and, in some cases, the entire

rear section strengthened.

In some cases there is a duplication of drives,

such as the tachometer drive, to connect instruments

located at separate stations.

The accessory section provides a mounting place

for the carburetor, or master control, fuel injection

pumps, engine-driven fuel pump, tachometer

generator, synchronizing generator for the engine

analyzer, oil filter, and oil pressure relief valve.