Why Second Officer usually assigned for long-haul flight?

點解SO 多數飛15個中或以上既機?

First of all, what is long-haul, short haul?

https://simpleflying.com/long-haul-flight-minimum-distance-guide/

https://avianet.ge/post/which-flights-are-considered-long-haul-flights

Taking reference from aviation related websites, there is no consistent definition.

However, I have tried to understand why CX need the SO for the so-called long hour flights – 仲要係 ‘Ultra Long Range Operations’ (ULR).

The reasons is complying to CAD 371 – The Avoidance of Fatigue in Aircrews 

The CAD 371 contains the standard provisions for Hong Kong Operators regarding approved flight time limitations.

Rightly, the  Cap. 448C Part 4 of the Air Navigation (Hong Kong) Order 1995 has requirements in addressing the fatigue of crew, when an aircraft registered in Hong Kong for public transportation OR for air transport undertaking. The CAD 371 is some documentation (standard provisions) for HK registered operators.

In CAD 371, the definition – ‘Augmented Crew’

The boarding of extra Flight Crew members for the purpose of providing relief at the controls. When four pilots are boarded for flights with a scheduled sector time of 15 hours or more, a maximum of one pilot may hold a P2X rating. When three pilots are boarded for flights with a scheduled sector time of 12 hours or more each pilot must be P1 rated on type.

Referring to CAD 54, the P2X is the Cruise pilot.

Notes:　the P2X rating holder, who is seated at the flying controls and discharges his duties
as an in-flight relief co-pilot, may log the corresponding on-seat flight time as P2X. The time spent outside
the control seat for rest purposes cannot be recorded as flight time for licensing purposes.

知多一點點:

CAD 371 – Section 7 是Definition, 有好多有用既詞彙值得知道:

7.10 ‘Flight Crew’

In relation to an aircraft means those members of the crew of the aircraft who undertake to act as pilot OR flight engineer.

7.15 ‘Long Range Operations’ (LRO)

An operation by a Two Crew Aircraft that includes a Sector with a Scheduled Sector Time greater than 10 hours but not greater than 16 hours.

7.35 ‘Ultra Long Range Operations’ (ULR)
An operation by a Two Crew Aircraft that includes a Sector with a Scheduled Sector Time greater than 16 hours, and where four pilots are boarded so that In flight Relief can be provided.

7.24 ‘Scheduled’

‘Scheduled’ is an intended future plan of what is intended to happen.

7.25 ‘Sector’ & ‘Sector Time’
The time between when an aircraft first moves from its parking position until it next comes to rest, after landing, on the designated parking position.

這就解釋點解 Block on, Block off (Chock on – Chock off) time 重要~