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 OFFPowered by ScribeFire.
