Date: 201604
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Weather elements / visibility: Cloudy; 10
Light: Daylight
Aircraft: Gulfstream IV / G350 / G450
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class D ZZZ
ATC / advisory: TRACON ZZZ
Reporter organization: Corporate
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction; Communication Breakdown
Second reporter function: Pilot Flying; Captain
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: General None Reported / Taken
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: During vectors for an ILS approach our aircraft was sequenced to follow a regional jet. After realizing that our aircraft was in a better position to be first on the approach the controller changed the order; had us keep our speed up and vectored us onto a base leg for the ILS. The subsequent intercepting vector however carried us through the localizer (most likely due to our higher than normal speed) and as we continued our turn to re-intercept the localizer I noticed the captain had descended to 2;400 MSL (100 feet below our vectoring altitude). I immediately brought this to his attention; he responded; and I then continued to complete the appropriate checklist.  After the completion of that checklist I began looking for the airport. It was then that I realized the captain had descended to 1;800 MSL and had not yet re-intercepted the localizer. Since we were VMC; knew the environment extremely well; and also knew there were no obstacles in the area I decided (wrongly) not to press the issue with the captain. Fortunately in approximately 10 to 15 seconds I visually picked up the airport and asked the controller for a visual approach which he immediately granted.The ceiling at the time was 1;700 broken which would be approximately 400 feet below the vectoring altitude and I believe the captain wanted to descend below that to conduct a visual approach. What we obviously should have done was to remain at the vectoring altitude and conduct the approach normally or possibly request a contact approach.


Second narrative: Approach Control was vectoring our flight for an ILS to runway 22. We were advised that we were number 2. The flight that we were to follow was observed on the TCAS and seemed to be an unusually long distance for our flight to be number 2. We made no comment to approach control. Do to the expected long vector; I had not configured the aircraft for the approach and was descending to the assigned altitude of 2;500 in a slow descent. As we were abeam of the final fix the controller advised us that we were now number 1 and to keep the speed up. I immediately started to configure the aircraft and increased my rate of descent. We were clear of clouds at this point with good visibility and no clouds below (good ground contact). The vector took us through the localizer to the East. I turned to a heading of 260 to capture the localizer. At this point we approached a cloud layer. As we had a full deflection and ground contact I started a descent to remain clear of the clouds and to keep the ground contact to an altitude of 1;800. At this point we were able to see the runway and reported the airport in sight. We were then cleared for a visual approach to runway 22 and continued on the localizer and glide slope for a landing on runway 22.


Synopsis: The flight crew of a G-IV reported descending below the assigned altitude after flying through the localizer in VMC conditions.