Medium Transport; Low Wing; 2 Turbojet Eng
Flight Crew Air Transport Pilot (ATP)
Other / Unknown; Situational Awareness
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance
Human Factors
Descent below required altitude while shooting the ILS approach.We were given a heading to intercept the localizer. The heading was at an angle that we would have crossed DERAG on the intercept. We were IMC at times. I recommended to the Captain (CA) to select green needles since we were in heading mode instead of having his FMS needle visible so he could arm LNAV to arm the LOC; but this did not happen right away. As we got closer; our clearance was on heading (can't remember) to maintain 3;000 until established; cleared for the ILS approach; report once established inbound. We were outside of DERAG; when the Pilot Flying (PF) started turning the heading bug to intercept the course inside of DERAG. Once established; the PF started descending to an unknown altitude as the altitude select was still set to 3;000 when the PF selected VS 600 fpm. As I was attempting to query the PF on why he was descending; Shannon Approach asked us about our altitude (now 2400 ft and still descending) and we were asked if we were already descending on the glideslope. I was directed to let Shannon know that we were; and I also advised them that we were established inbound. However; this was not the case; because we should have been at 3;000 feet; and allowed the GS to intercept accordingly. Spoke to the PF once at cruise on next leg about this issue. We looked at the approach together. I brought up the fact that since I have been flying with this pilot; he always descends on an ILS on stepdown by using VS instead of allowing the GS to intercept even when there is NOT a mandatory crossing restriction.  The PF had no response. The PF understood he should have been at 3;000 feet and acknowledged he made a mistake. In addition; I brought up the CRM issues that I have only most recently been experiencing with this particular pilot whereby this pilot either will not acknowledge me and/or not respond to my questions; suggestions; etc.
G-IV First Officer reported poor CRM practices of the Captain she was flying with which resulted in descent below assigned altitude during approach and a possible track deviation during departure.