Yes, a jetliner’s engine can be started by a technician or ground personnel on the apron. This is sometimes necessary for maintenance purposes or in case of a fault prevents the start valve from being electrically actuated from the cockpit.
Here’s a video of an Airbus A321neo’s LEAP-1A engine being started:
[Warning: Loud Noises]
In this above, the APU has already been turned on to provide the electrical power and bleed air necessary to start the engine. The ramp agent inserts a ratchet into a small access hatch in the engine nacelle where a keyhole can be turned to mechanically open the starter valve so bleed air from the APU can be fed into the engine.
The following YouTube description was provided with the video:
Electrical power is needed, which is being provided by the APU (auxiliary power unit). Aircraft has to be configured in a proper way so a sequence of events can occur. Not only are we using electrical power from the APU but also pneumatic power. Bleed air is taken from the APU in order to start the engines via the pneumatic starter. Before air gets to the starter it needs to pass through a starter valve.
[…] That brings us to the demonstration you’re witnessing. You see me walking up to the engine with a 3/8 inch ratchet and extension. This aircraft had a malfunctioning starter valve which was placed on MEL (minimum equipment list), this means I have to manually turn the valve to the open position in order for the air to be fed to the starter from the APU.
My colleague is standing at the front with a headset in constant communication with the pilots. They trigger a sequence of events in order to start the engine, and I am given hand signals to turn the valve to open position. The engine begins to spool up and reach the proper RPM. Once it has reached sufficient speed and fuel is added and ignition occurs, the engine begins to be self sufficient. You can hear it in the video with that deep rumbling sound.
At this point the starter needs to be cut out and the engine no longer needs bleed air, at which point my colleague will give me the signal to close the starter valve. Once this occurs I step away safely from the engine.
Here’s a schematic of what the component might look like:
[Source: Aviation Stack Exchange]
In essence, a mechanical override switch in the engine nacelle allows the engine startup process to be initiated on the tarmac without any inputs from the cockpit. However, as explained, certain conditions must be fulfilled, depending on the airliner model, such as having electrical and pneumatic power made available. Note that the Boeing 787 only requires electrical power for engine startup.
Addendum: In the past, propeller engines were hand started, and someone had to swing a propeller blade and immediately get out of the way to avoid turning into minced meat. This was was known as “hand propping”. Early aircraft had weaker engines and lacked a starter device—apparently to save weight—and literally needed a hand to start.
This can still be done in today’s day and age, but it’s generally discouraged, given how dangerous it is. Technically the engine can be started without a pilot in the cockpit, by setting the brakes and the ignition switch to ‘ON’, but practicality speaking, it’s a team effort: a pilot needs to coordinate with the hand starter to ensure a safe startup.