A Guide to Airliner Simulation: Part Four - Pushing Back
In the previous tutorial we completed the cockpit preparation and programmed the Control Display Unit (CDU) with the route and performance data. We have been given a departure runway of 27R and the BPK7F SID, which we have entered on the CDU. Next up is to look at the take-off.
Flight deck preparation
The runway at Heathrow is an impressive 12,802ft (3900m) so we are not limited by the take-off distance available (TODA), enabling us to use the minimum flap settings for the departure. For example, on a Boeing 737, we can set flaps 1, 2, 5, 10 or 15 for departure. For short runways, we would use flaps 15 to reduce the take-off run, but climb-out performance in the second segment (when the gear is retracted but the flaps are still extended) would suffer due to the increased drag. On long runways we can use less flaps, which will result in a longer take-off run but we have better climb performance in the second segment.
We should also be aware of the geography around the airport. The Minimum Sector Altitude (MSA) is the lowest altitude we can fly with 1,000ft clearance above all objects within a 25Nm radius. From the charts we can see the MSA around Heathrow is 2,200ft in the northwest quadrant and 2,100ft for the remaining sectors. This tells us there are no major obstacles around the airport, although it is worth noting that the city of London is to the east. If anything happens during take-off that would force us to divert off our route, this is the area to avoid. So, we have a long runway and no significant terrain; the aircraft is 20 tonnes underweight, so we can use a reduced power take-off. This is common practice with airline operators to increase engine life and save fuel.
Verify the Zero Fuel Weight (ZFW) entered on the CDU matches the Operational Flight Plan (OFP). A wrong ZFW may result in wildly incorrect takeoff speeds.
The Thrust Limit page is where we select how much power the engines should generate on take-off. We covered this topic in the previous tutorial but to summarise, there are two methods we can use. The first is to de-rate the engines. Smaller engines generate less asymmetric thrust in case of engine failure, so we can use a lower Vmc (minimum controllable speed with an engine out). This is particularly useful on slippery runways where lateral control is reduced. So, we can actually increase the take-off weight by using less power. The second method is to use an assumed temperature take-off. In this case, we tell the engines the ambient temperature is higher than the actual outside air temperature, which will make the powerplants generate less thrust. We can still command full power from the engine if needed but the downside is we cannot take advantage of the lower Vmc. In Airbus aircraft, assumed temperature on takeoff is known as FLEX (Flexible Temperature).
Finally, we need to set the flaps and stabiliser trim. This is the elevator trim setting, which depends on our weight and the position of the payload. We would normally get this from a load sheet or OFP (Operational Flight Plan), but most flight simulator packages automatically give us this value by clicking on the Centre of Gravity (CG) button on the CDU. We also need to check our thrust reduction altitude and the acceleration height. This is the altitude where the engines reduce thrust to climb power and we start accelerating to flap retraction speed. This is usually between 1,000 and 1,500ft. We now have all the data needed to enter the Vspeeds on the Takeoff Ref page on the CDU, V1 (decision speed) Vr (rotate) and V2 (take-off safety speed). The CDU pre-flight is now complete and we can set the Mode Control Panel (MCP) with the initial climb-out altitude – 6,000ft which is the altitude restriction of the SID, set the heading bug to the runway heading (271 for Runway 27R) and the speed window should be set to V2 or 142kts.
This is now a good time to revise the take-off procedure. For a normal departure, we should make note of our departure runway, SID, transition altitude and speed/altitude restrictions.
However, we also need a plan of action in case of a failure on take-off. Usually, the captain will run through scenarios and what to do in case of an emergency. If we get any abnormal indications or warning below 80kts, we should abort the takeoff.
This is known as a low-speed abort and is usually a safe procedure. Above 80kts but below V1 this is called a high-speed abort and in this case, we should only abort for major failures such as a fire warning, engine failure, wind shear warnings or if the aircraft is unsafe or unable to fly. Above V1, an abort is no longer an option and we need to take the problem with us into the air. If this happens, we should have a plan of action to return for a safe landing. We should know our MSA and holding patterns on the SID. We should also set up the course and frequencies of ground-based navaid in case of loss of RNAV navigation. Finally, we should also have our take-off alternates ready. It is not always a good idea to bring a badly damaged bird back to a populated airport such as Heathrow. Sometimes the wise course of action is to divert to a more remote field in case we are unsure of a safe landing. For example, Stansted would make a good take-off alternate for Heathrow.
We can now start the Auxiliary Power Unit (APU) to get some air flowing around the cabin while the passengers are boarding. This is a good time to run through the before start checklist. Actions to be taken depend on the aircraft type and the Standard Operating Procedures (SOPS) of the airline but the steps are very similar. The fuel pumps should come on and anti-collision lights should be on – this tells the ground crew the aircraft is under its own power. We can also pressurise the hydraulic system by switching the electrical demand pumps on. When pressurising the hydraulics in the real example we need to make sure the ground crew are clear of the control surfaces as they can move when they become pressurised. The autobrake should be set to Rejected Take-Off (RTO). In case we need to abort the take-off, RTO will apply maximum braking automatically if the thrust levers are brought to idle on the take-off run.
Once refuelling is complete, we can switch on the seatbelt signs and when boarding is complete, shut the doors, disconnect the jetway and contact ATC to request push and start.
The syntax for this is “Ground, Aircraft callsign at gate 122 with information Golf, QNH 998, requesting push and start”. We will be told we are cleared to push and start and sometimes given the direction the tail should be facing, ie east or west. Once we have received the pushback clearance, we can contact the ground crew to hook up the pushback tug and push pack. After getting the green light for pushing back, we release the parking brake and the tug starts rolling us back.
It is normal to start the engines during the pushback process. On most airliners, we use bleed air from the APU to spool the engines up. You may have noticed the airflow in the back of the cabin stops as the aircraft starts rolling back. This happens when the pilots turn the packs off, diverting the airflow from the cabin to the engines. So basically, the start-up process is: packs are turned off (in Airbus this happens automatically). Number 2 engine (right engine) is usually started first as it is the furthest from the jetway. When we engage the engine start switch, bleed air from the APU is diverted to the engine and we should observe the high-pressure compressor or N2 start rising, followed by an increase in oil pressure. When N2 reaches around 25%, we introduce fuel into the engines by placing the fuel cut-off lever to the idle position. We should now see the temperature in the engine core start rising. This is known as Exhaust Gas Temperature (EGT) or Turbine Inlet Temperature (TIT) depending on aircraft type. Turbine engines are vulnerable to overheating during engine start-up as they are primarily cooled by the airflow through the engine core. This is why we spin them up to 25% N2 before adding fuel. If we introduced fuel at lower RPM, there wouldn’t be enough airflow through the engine core to cool it down and it could overheat, potentially destroying a multimillion-pound engine in a few seconds. During the start-up process, we need to carefully monitor the temperatures and oil pressure.
At around 60% N2 the engine becomes self-sustaining, EGT should peak and then start stabilising. At the same time, the N1 (low-speed compressor) should start building up speed. For example, in a 737 at idle, the engine should stabilise at an N1 of around 20%, EGT at around 400⁰C and an N2 of 60%. Once the engine is stable we can start number 1.
When both engines are stable, we can run through the before taxi checklist. Again, this is specific for aircraft types, but most of the actions we need to perform are common.
The engine generators and packs should be switched on, and the APU turned off. Make sure the engine generators are online before turning the APU off or the aircraft will lose electrical power! Other checks we need to do is turn the probe heat on and set the engine igniters to continuous. This fires a spark into the engine and reduces the chances of a flameout during take-off. We should also check the flight controls and we can monitor their position on the lower EICAS display.
We set the flaps to the take-off setting (flaps 1 for this one), verify the stabiliser trim matches the value on the CDU and the aileron and rudder trims are neutral. Next, press ‘Recall’ to check we haven’t got any cancelled messages. Finally, the taxi and turnoff lights come on and if we are flying from an airport with ground surveillance radar, the transponder should be switched on to TA mode. We are now ready to call up ground control and request taxi.
On Boeings, we can enter our transition altitude (TA) on the Perf Init page on the CDU. As we pass the TA, the altimeter setting on the Primary Flight Display (PFD) turns yellow, to remind us to switch to a standard altimeter setting.
Finding your way to the runway
Like airways, taxiways are identified by alphanumeric characters. For example, we were given the following clearance: Taxi to Runway 27R via taxiway K, B, Lima 13, A, to holding point A3. This means take taxiway Kilo to taxiway Bravo to intersection 13 (Link 13), then pick up taxiway Alpha to holding point A3 for Runway 27R. While most simulators will allow us to cheat by turning on progressive taxi, I recommend you write down the taxi clearance and try to navigate your way using an airport diagram. It is challenging to start with but ultimately rewarding.
Preparing the aircraft for departure is a very busy time on the flight deck but we are now ready to make our way to holding point A3 and get airborne, which we will cover in the next issue.
By Richard Benedikz