Airbus 330-200 Engines

Prior to taking the air again, I think that it’s now justifiable, after 10 successive legs, to wonder how we actually start the engines of our A330-200 aircraft.

Starting the engines of an Airbus airliner is easy. The main reason for this is that engine start, like many other Airbus systems, is a partial automatic procedure.

 

To explain the procedure, I propose to start from the 'cold and dark' cockpit configuration. This is a simple procedure indeed, but we will take some time here to wander from the point and focus on some of the concerned systems.

 

First of all, we have to supply the aircraft with electrical power.

Before engine start, the aircraft can supply its own DC electrical power coming from onboard batteries (BAT 1, BAT 2, APU BAT). The DC electrical system supplies 28 Volt DC and it is mainly used to start the APU, or supply the aircraft when other sources are unavailable. When their voltage goes below a certain level, the batteries will charge from the AC electrical system driven by the engines – just like a car battery ! The AC electrical system provides 3 phase 115/200 Volt, 400 Hz AC provided by engine driven generators installed on each engine. A further generator installed on the APU is used to provide AC power before engine start (and after engine shutdown) or in case of loss of one or the two engine generators. And finally, in case of total loss of electrical power during flight, the aircraft can be supplied by an emergency generator driven by the RAM air turbine, using the high speed air flow to provide power a bit like a wind engine.

APU and engine generators will automatically work once the concerned engine runs, if their corresponding switch on the overhead ELEC panel displays no light. In this configuration, APU generator will also automatically stop once engine generators power become available (or external power is connected). Pilots may still disconnect any generator from the electrical system by switching it off ("OFF" will illuminate). 

On the ground, the aircraft can also be supplied by external power.

We’ll use the aircraft onboard system to start the engines.

 

 

The 3 batteries are switched on.

By the same time, the avionics and the main panel CRT screens come alive.

 

It’s now time to switch on the fuel pumps, but let’s see first how is roughly working the fuel system.

On the A330 type, the fuel is contained in one center and two wing tanks. Because of in flight structural reasons, the center tank is usually filled last and used first. The wing tanks are divided into inner and outer cells which are connected by transfer valves. The engines are fed from the inner cells. There are two main fuel pumps and one standby pump in each inner tank. The main pumps all work in normal operations. If a main pump fails or is switched off, the standby pump takes over. The two center tank pumps work as long as there is fuel in the center tank. Fuel transfer from the center tank to the inner cells is controlled by inlet valves, which top up the inner cells while there’s still fuel in the center tank. After that, once one inner cells goes below a preset level, transfer valves open to allow fuel from the outer cells to flow into the inner cells. In addition to that, a crossfeed valve connects the left and right fuel systems, allowing both engines to be fed by both wing tanks, or one engine to be fed by both wing tanks in case of engine failure and single engine situation. Fuel transfer and valve operations work automatically during flight. The Flight Management System computes the best fuel load balancing in real time for the best aircraft performance. This was once the job of the Flight Engineer. Nowadays Airbus pilots are however always informed by ECAM messages, and can still manually operate on fuel transfer using the Overhead's  FUEL panel.

Now that we’ve demonstrated that fuel pumps may be useful for the engines, let’s switch them to ON ! 

 

 

On the picture, left and center systems have already been set to ON. You'll notice that the center tank pumps show 'FAULT' : this is normal, since there's no fuel in the center tank needed for this new short flight.

 

The next step is to start the APU. The APU (Auxiliary Power Unit) is a small, single spool (N) jet engine located in the aircraft tailcone. APU, which is now common with every modern airliner, allows the aircraft to be independent of external pneumatic and electrical power supplies. The APU can then provide bleed air for engine start and cabin air conditioning and has its own generator to provide AC electrical power (see above). APU can be started on the ground as well as in flight. Starting the APU works almost the same as switching on a lamp. Switching ON the APU MASTER SW button (blue "ON" illuminates) arms the APU system for automatic startup sequence and opens the APU air intake flap. Just below, the START button will initiate automatic startup. When completed, a green 'AVAIL' will illuminate and will also appear on the upper ECAM, meaning that APU is available.

 

 

 

 

The latest image shows both upper and lower ECAM after APU start.

 

Now that the APU is running, we can use its bleed air for engine start. High pressure air (as well as electrical power) is needed to start the main engines. Bleeding air into the engine will initiate the spool rotation until it reaches a preset rate at which ignition will occur.

Some technical background about the jet engine is not superfluous here before going further.

 

The turbofan jet engine which is powering our aircraft, as well as most of nowadays airliners, is an evolution of the twin spool turbojet engine.

A spool is a couple compressor – turbine mounted on the same shaft, and then rotating at the same speed. The compressor and turbine are in most cases a succession of stages. Each stage includes a rotor grid which is integral with the shaft. The rotor grid is a disc fitted with a great number of rotor blades. In modern engines, the rotor blades pitch is variable as a function of the air flow to optimize performance. Each rotor grid is followed by a stator grid, whose purpose is to straighten up the flow to the axial direction of the engine. On most modern engines, the stator vanes pitch is also variable. Each grid will modify the air flow direction and speed.

The intake air will cycle throughout the engine. On the basic single spool engine (like the one that powered the first generation Boeing 707 or DC8), the cycle includes the compression, combustion and expansion. During the cycle, air pressure, volume and temperature change, linked together following thermodynamics laws. The compressor will make the first step. Air pressure and temperature will increase. In the combustion chamber, which is inserted between the compressor and the turbine, compressed air will be mixed with fuel by a complex mixing system. The fuel combustion will considerably increase gasses temperature as well as volume, while pressure will remain almost constant. And finally, gasses will first expand in the turbine driving the compressor, and then complete their expansion in the nozzle, providing the rear thrust that will propel the aircraft in the opposite direction, as result of the well known reaction law. The temperature will decrease during the expansion phase, giving the exhaust gas temperature (EGT) which is an important engine parameter to be controlled by the pilots, with the turbine materials thermic constraints in mind. A dramatic and sudden increase of EGT will result of an engine fire.

The twin spool turbojet (dating back to the sixties) has two concentric shafts rotating at different speeds. A good example is the PW JT8D engine, which powered many aircraft models including B727, B737-200, DC9 and MD80.

The low pressure spool (N1) includes the low pressure (LP) compressor and the low pressure turbine.

The high pressure spool (N2), located between the LP compressor and the LP turbine, includes the high pressure (HP) compressor and the high pressure turbine.

N1 spool rotation rate is often the main driving parameter of the engine, while N2 plays a part during engine start. As an example, take off rate on the CFM56-3B1 (powering the B737-300 and A320 families) will bring N1 to 5175 RPM and N2 to 14460 RPM. Meanwhile, N1 and N2 rotation rates will not be directly displayed as RPM (revolutions per minute) in the flight deck but as a percentage of a maximum rate. Jet engines are characterized by two maximum rates: the maximum take off performance (TO), which is the highest, but limited to a 5 minutes time interval; and the maxi-continuous performance (MCT), which is lower but unlimited in time. Because the gauge range exceeds 100% for the engine rate, this is probably the MCT constant that is used as reference.

The intake air is sucked in by the LP compressor. Unlike on the single spool engine, a part of the intake air will be derivated during the LP compression phase and will NOT be driven to the combustion chamber, therefore bypassing the normal cycle. There are then two air flows on such engine: the cold stream that bypasses the hot gasses generator and the hot stream which goes through the normal cycle. This introduces a new parameter: the bypass ratio, which is defined as the ratio between the derivated air quantity and the normal cycle air quantity, and can then be seen as

 

                        cold stream air quantity

Bypass ratio =  ──────────────

                        hot stream air quantity

 

For a twin spool turbojet, the bypass ratio is roughly equal to 1. Note that for a single spool engine, it will be equal to zero.

We can demonstrate that increasing the bypass ratio will increase engine performance, reduce fuel burn as well as engine noise.

The turbofan engine is a direct application of that concept. In a turbofan, the first stage of the LP compressor, which sucks in the total intake air, is an oversized rotor grid, called the fan. The first turbofans powered the B747, DC10 and L1011 Tristar in the seventies, to be followed by the Airbus A300-B1.

The bypass ratio, as well as the compression rate are greatly improved. The bypass ratio of the first turbofans was close to 5. The big turbofans of the new generation have a bypass ratio equal to 6, with a fan diameter close to 3 m. The General Electric/Pratt & Whitney GP 7200 and Rolls Royce Trent 900, designed for the A380, are currently the biggest and the most efficient engines on the market.

The global thrust provided by such engines is mainly provided by the fan. The fan produces from 70 to 75 % of the engine thrust. It can then also be seen as a streamlined propeller driven by a first generation twin spool turbojet. Besides, only the fan blowed cold stream air will be used by the reverser system.

Rolls Royce engines have a distinctive feature: they have 3 spools: N1 (LP), N2 (IP for Intermediate Pressure) and N3 (HP). In this case, the fan acts as the LP compressor on its own.

The following diagram shows a generic twin spool turbofan engine longitudinal section, where you'll easily locate many of the components that were introduced here.

 

 

Aircraft engine performance evolution always sees greater bypass ratio, greater pressions, greater temperatures, lower noise emissions, lower fuel burn and lower environmental impact. For example, higher temperatures in the turbine inlet (approaching 1500°C) are nowadays possible thanks to better materials as well as a complex cooling system allowing the blades to work above their melting point. The fan size, and therefore the bypass ratio, however, have a upper limit. The engine cannot grow indefinitely since it must still be able to be installed on the aircraft and the bigger is the fan, the greater will be its aerodynamic drag.

 

Now that we're more familiar with the jet engine – and if you are wondering whether we'll once reach Tel Aviv – let's continue with the startup sequence.

We'll perform an automatic start using the APU bleed air for both engines. Engine start usually takes place during the aircraft pushback.

Switching ON the APU BLEED (overhead AIR conditioning panel) will allow blowing high pressure air into the engine, and more precisely, into the High Pressure spool N2 (that would be N3 on a RR engine).

 

 

On the A330, engine #1 (left wing) is usually started first. Not difficult to remember anyway.

Checking APU BLEED is ON, we set the ENG START selector to IGN/START on the pedestal. Next, we set the ENG MASTER switch 1 to ON.

 

 

N2 spool will start rotating (monitor N2 rate increase on the upper ECAM). Once N2 reaches a preset rate, which is commonly 20% for every jet airliner, fuel is injected into the combustion chamber and ignition occurs, the complete engine cycle can start. While opening the fuel cutoff lever for each engine  is necessary on Boeing planes, fuel injection will be done automatically here at 20% N2 and pilots just have nothing more to do.

Once engine 1 is stable (monitor N1, EGT, N2 and fuel flow on the upper ECAM ), setting ON the ENG MASTER switch 2 will start engine 2 in the same way.

The following picture shows the E/WD (upper ECAM) once both engines are started and pushback is completed. ENG START selector is then set back to NORM mode. APU bleed is switched off, but we will leave APU running until we're airborne and above 10,000 feet. APU & APU BLEED should be started again after landing, so that the aircraft can still be powered after engine shutdown.

 

 

So, if we sum up, to start the engines of our A330 we just had to

 

1) set BAT 1, BAT 2, APU BAT to ON

2) switch ON the fuel pumps

3) start the APU

4) check APU GEN, ENG 1 GEN, ENG 2 GEN "no light"

5) set APU BLEED to ON

6) set engine start selector to IGN/START

7) set ENG MASTER switch 1 to ON

8) once engine #1 stable, set the ENG MASTER switch 2 to ON

9) once engine #2 stable, set the engine start selector back to NORM

10) set APU BLEED to OFF



Switch Off:

The following sequence

 

1) set ENG MASTER switch 1 to OFF

2) set ENG MASTER switch 2 to OFF

3) switch off the fuel pumps

4) switch off APU BLEED

5) set APU MASTER SW to OFF

6) switch BAT1, BAT2, APU BAT to OFF

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