If you are a frequent flyer, you have most likely noticed aircraft being pushed back from the gate with the help of a behemoth ground vehicle with fairly large wheels and a mechanic walking close to the nose gear. This pushback procedure is peculiar to many aircraft, new or old. With that said, have you ever thought why these advanced machines should be pushed back rather than move with their own power?
Before answering the above question, let us cover more ground on aircraft pushback.
What is a pushback?
As the name implies, when an aircraft is parked in a nose-in parking position, it should be moved to a position where it can move forward with the help of its own engines. This specific movement is done by pushing the aircraft backward, naming the procedure pushback.
A powerful ground vehicle named tow tug undertakes the pushback via a tow bar attachment to the aircraft nose landing gear. This is more like towing a broken car, but in the opposite direction.
In a typical pushback from the gate for departure, three ground crew members are involved:
Tow tug drivers
Aircraft mechanics, who communicate with the cockpit via an interphone
Auxiliary ground crew members, who carry a fire extinguisher and wheel chocks
Main steps of a pushback
While pushing back airplane sounds quite straightforward, there are a number of steps involved in the procedure. Not only does it involve a ground crew supervisor but also aircraft mechanics and pilots. So, let’s take a look at the 10 crucial steps of a push back:
Once the aircraft is ready for the departure, ground crew (usually the aircraft mechanic) disconnects nose wheel steering (NWS) by moving the steering bypass lever in the steering deactivation box to deactivated position and inserting the bypass pin.
Connect the bar to the tow fitting in the nose gear.
Then connect the tow tug with the towing bar.
Remove the external power unit and all the other attachments to the aircraft including the aerobridge/boarding steps.
Ground crew communicates with the pilot and informs when the aircraft is ready for the pushback.
When the pilots grant clearance and release parking brakes, the ground crew personnel at the intercom signals the tow tug driver to commence pushback.
During the pushback, ground personnel maintains communication with the cockpit and grant clearance for starting aircraft engines.
When the aircraft reaches desired position, ground personnel inform the pilots that the pushback has been completed and it is time to apply parking brakes.
When the brakes are applied, tow tug and tow bar are disconnected from the aircraft and taken back.
The mechanic removes bypass pin and walks away. The mechanic signals the pilots that the bypass pin has been removed. Then the aircraft can move forward with its own power.
Take a note!
The aircraft weight should be less than Maximum Ramp Weight (MRW) to conduct a pushback.
All passengers and cabin crew should be seated during the push back.
Applicable towing bar has to be used for each aircraft type. Usually, tow bars can be used for many aircraft models.
No engine can exceed idle thrust during the pushback.
The tow bar should be equipped with a damping mechanism, a calibrated shear pin for disconnection at high loads, and another calibrated turn shear pin for disconnection at higher angles.
In each pushback, the pilot in command in the cockpit should request RTF clearance from the ATC.
There are specific towing limitations for each aircraft configuration. For example, an A330 towing for gate to gate transfer should carry no payload and allow only 21,000kg of fuel to be onboard.
Apart from the usual pushback operations you have came across, aircraft maintenance personnel perform special pushbacks in various occasions:
Changing the aircraft gates and moving from a gate to a remote bay or vice versa.
Moving an aircraft out from a hangar.
In these occasions, two additional ground crew members (wing walkers) are used to monitor the wingtips to ensure adequate leeway from other aircraft and structures.
What could possibly go wrong in a pushback?
A pushback seems undemanding: connecting the tug with the aircraft and moving it around the tarmac area seems easy enough. But there is more gravity in this procedure. The entire pushback crew is responsible for the safe execution of the aircraft pushback.
The ground crew supervisor is responsible for the entire task from greeting the pilot to the ground vehicle disconnection from the aircraft nose landing gear. Ground crews must be aware of the hazards that come with moving a huge machine around and supervise the whole procedure with full attention. Here are some things they have to watch out for:
Exceeding the towing angleExceeding the towing angle can lead to undue stress on the nose landing gear of the aircraft, and in severe enough cases can lead to damage to the nose landing gear linkage and possible collapse of the gear.
Aircraft colliding with objectsWhile ramp operations are limited to low speeds, in situations such as inclement weather or simply a bad day where situational awareness is low, the possibility of aircraft collision with either other parked aircraft, lighting equipment, a terminal building or other support vehicles exists.Collision with any object poses serious risk to cause invisible or serious damage the aircraft structure. Any malfunctioning or damaged objects on an aircraft can lead to fatal accidents during any phase of a flight.
Ground personnel injuries There have been a variety of instances in which due to lack of attention or other human errors, a ground handler has been pinned under the airplane wheels, grazed or injured by various aircraft parts. These incidents usually happen due to errors in communication, overlooking procedural standards or negligence. To prevent this from happening, ground crew should be regularly trained and informed of the risks during the push back.
Can’t a pilot push back by themselves?
The answer is yes, they can. This is a procedure referred to as a power back. During a power back, the pilot can use the aircraft’s own engine power to move the vehicle to a desired position.
Naturally, since a power back only required onboard crew and the aircraft’s own power to move, this method of a push back is more man-power efficient. The airplane does not need to rely on tow tugs, wing walkers and other staff involved to get across the tarmac. However, a question arises, why do we see power backs so rarely nowadays?
Why power back pushbacks are no longer an interest?
Power back pushbacks were more predominant in aircraft with tail-mounted engines rather than wing-mounted engines as ingression of Foreign Object Debris or FOD was unlikely due to the higher elevation of the engines. In some rare cases, aircraft with wing-mounted engines have pushed back with reverse thrust.
Power-back pushbacks are seemingly to be a win-win solution for both the pilots and ground crew in the first place. It could cut down the cost of maintaining an entire tug fleet with thousands of man-hours. But, this method was considered prejudicial and hazardous by the authorities. Let us find out how it endangers the safety before the taking off.
Powerful powerplants equals more hazards
Let’s take a look at some of the potential hazards created by the powerful modern jet engines on aircraft during a power back.
Engine damage by foreign objects
Modern aircraft are fitted with extremely powerful engines that suck in tons of air in seconds. Reverse thrust at idle speed is not sufficient enough to push the aircraft backward. Pilots have to increase the thrust of the engine, hence creating more suction. Apart from the suction, diverted airflow can lift up Foreign Object Debris (FOD) on the ground creating a suction hazard to the engines.
Collision with non-aircraft objects
The location of the powerplant too plays a pivotal role in a power back pushback. Modern airliners have their engines mounted under the wing close to the ground. Reversing the engines at this close proximity to the ground could possibility airborne anything close to the intake creating a suction and collision threat.
Due to powerful suction, various airborne objects can be flung at structures like terminal building windows, transportation busses, ground vehicles, billboards, and other equipment damaging them and passengers or personnel.
Especially when parked at an airport gate, huge noise generated from the perturbed airflow is definitely a threat to the terminal building. Light objects can take flight and collide with the aircraft and surrounding objects.
More importantly, noise footprint of the airports is a major concern for the occupants and residents close to the airports, especially at night. In power backs, reverse thrust will create a significant racket. In fact, it will generate more noise than an aircraft moving forward with its own power, all due to the reversal of airflow. Even if the aircraft is capable of performing a power back (like DC-9, MD-80, and B727), local procedures of the airport may forbid due to noise limitations.
In addition to the above, modern day jet engines are calibrated for unidirectional airflow through the core of the engine. Deploying reverse thrust without forward movement of the aircraft can disrupt this airflow and lead to compressor stalls.
High fuel consumption
With increasing fuel prices and concerns about carbon emissions, airlines are trying to keep their overall consumption at a lower value. For power backs, both engines should be fired at the same RPM to create symmetric thrust that will definitely entail a higher fuel consumption rate.
An airplane resting in a hangar with its engines removed is just a deadweight and pilling up expenses. Operation of thrust reversers clocks up engine hours and the number of cycles in the reverser system, which will reduce the Time Between Overhauls (TBO).
Pilots are blind on their back
In a usual pushback with the towing bar connected to the nose landing gear, pilot inputs are disconnected via a steering bypass pin inserted into the steering deactivation box in the nose landing gear. This is because a cockpit view has more blind spots and the flight crew are unaware of what is going on behind them.
Can propeller aircraft use reverse thrust to push back?
The answer is also yes! But, regardless of being a propeller-driven aircraft, it has to face all the aforementioned drawbacks. Modern turbo-props are capable of reversing the propeller pitch to create reverse thrust. On the ground, with adequate engine RPM, a reversed-pitch propeller could push back the aircraft.
As seen in the ATR-72 in above image, in most cases a high-wing mounting position is peculiar for most propeller engines- to keep the propeller clear from the ground. This feature makes power back pushbacks a little more convenient for turbo-props when compared to turbofan engines.
Now we have summarized the most important points you should know about an aircraft push back procedure. Here are some of the main points to take away from the article:
An aircraft pushback is a method to move an aircraft from the gate or a parking spot to another desired location. A pushback involves a series of steps that are executed both by the pilots and the ground crew. You will also often see vehicles called tow tugs helping the aircraft move backwards. However, you should know that the aircraft can also move by itself using reverse thrust but the method is not used as widely due to many drawbacks both to the aircraft as well as the environment.
Have you learned something new? Let us know in the comments!
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Jet pilot @NASA
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