EC REG 965/2012 states (my bold):
ORO.FC.A.201 In-flight relief of flight crew members
(a) The commander may delegate the conduct of the flight to:
(1) another qualified commander; or
(2) for operations only above flight level (FL) 200, a pilot who complies with the following minimum qualifications:
(i) ATPL;
(ii) conversion training and checking, including type rating training, in accordance with ORO.FC.220;
(iii) all recurrent training and checking in accordance with ORO.FC.230 and ORO.FC.240;
(iv) route/area and aerodrome competence in accordance with ORO.FC.105.
CAT.GEN.MPA.105 Responsibilities of the commander
(a) The commander, in addition to complying with CAT.GEN.MPA.100, shall:
(1) be responsible for the safety of all crew members, passengers and cargo on board, as soon as the commander arrives on board the aircraft, until the commander leaves the aircraft at the end of the flight;
(2) be responsible for the operation and safety of the aircraft:
(i) for aeroplanes, from the moment the aeroplane is first ready to move for the purpose of taxiing prior to take-off, until the moment it finally comes to rest at the end of the flight and the engine(s) used as primary propulsion unit(s) is(are) shut down;
(ii) for helicopters, when the rotors are turning;
(3) have authority to give all commands and take any appropriate actions for the purpose of securing the safety of the aircraft and of persons and/or property carried therein in accordance with 7.c of Annex IV to Regulation (EC) No 216/2008;
As Captain Seamus J.P. Lyttle, (BSc, CEng, FCLIT, FRAeS) summarizes in a great book "So you want to be a Captain?:
1.2.7 Summary
Commander’s responsibility:
• The commander is charged with the safe conduct of his flight;
• It is the commander’s responsibility to make and take decisions – to command.
Responsibility:
• An onus or obligation to achieve a safe operation;
• Assigned by the operator;
• Charged by regulation;
• Discharged by proper execution of duties;
• Not re-assignable on to others;
• Cannot be rejected by the person to which assigned.
Overall responsibility includes:
Responsibility for the actions of subordinates – e.g. crew members;
Responsibility to assess the product or outcome of others’ activities, where these affect or impact on the basic responsibility assigned to the commander.
So my question is: Who's ultimately responsible, when another qualified commander (and I use the term commander on purpose) is at the controls?
I have my opinion, but would like to have yours'.