Have you ever heard what sounds like dogs barking as your Virgin America flight taxis to the gate after landing? No, it’s not a pack of hounds loose in the cargo hold (Virgin America, by the way, does not transport pets in cargo). What you’re hearing is the sound of the aircraft equalizing hydraulic pressure as the pilots use just one engine to taxi to the gate. Single-engine taxiing is another way Virgin America saves fuel and strives to be more environmentally friendly.
Each of our Airbus A320-family aircraft is equipped with two high-bypass turbofan engines, but when we can, we use one engine to propel the aircraft from the gate to the runway. About 10 minutes before the aircraft is expected to leave the gate, the pilots will start the auxiliary power unit (APU), which is used to start the engines and provide in-cabin power once the aircraft is disconnected from ground power. An engine on one of our A320-family aircraft takes about three minutes to warm up, so the pilots typically start one as the aircraft is pushed back from the gate. You may have heard the engine spool up when the tug releases the aircraft.
About three minutes before the anticipated take off time, the pilots will start the other engine and power down the APU. So for most of the trip down the taxiway, the aircraft will be powered by one engine. If air traffic control tells the pilots that the expected taxiing time is short, they may opt to start both engines. When both engines are warmed up, the aircraft is ready to take off.
On landing, this process is reversed. When the aircraft reaches the taxiway at the end of the runway, the pilots may shut down one engine if they expect a long taxiing time to the gate. And this is when you might hear the “barking dog” noise – particularly when the pilot shuts down the engine at the gate — as the aircraft seeks to equalize hydraulic pressure.
So why do we single-engine taxi when we can? Our flight operations department estimates that the single-engine taxiing saves an average of 22 gallons of fuel per flight, and we operate about 160 flights per day. So over the course of the year, the number of gallons of fuel saved – and tons of carbon dioxide not emitted – begin to add up. Just as with Sharklets, this is yet another way Virgin America seeks to be as carbon efficient as we can be.