EMB ERJ 170/175 ER/LR
Human-Machine Interface; Training / Qualification
Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Aircraft; Human Factors
We were descending via the DOOBI Two RNAV Arrival into IAH.  Everything was briefed correctly; in range checklist was completed and no issues had occurred on the arrival.  We had anticipated that we would be given Runway 26L so the Captain (CA) had loaded that into the FMS earlier and I included it in my brief.  Right after we went over the BOPPR intersection ATC had cleared us for the ILS 26L into IAH.  I communicated to the CA that we would just fly the RNAV fixes all the way in and transition into NAV mode and bring up the ILS when we get closer.  I also communicated to the CA that I would preset the altitude selector to 2000 feet which was the final approach fix at JEPNI.  I accomplished that after I verbalized my approach plan.  I also stated that I would continue to fly the VPI (Vertical Path Indicator) down on the arrival and into the approach transition.Even though this seemed like a simple procedure and should not have been any task saturation or work load issues at all I still felt a little overwhelmed with the type of clearance given since I was not used to transitioning from an arrival to an ILS using just the FMS.  In the process; I completely lost track of what altitude I was supposed to be at on the approach as I was crosschecking the fixes and trying to determine at what point I would switch to the ILS frequency and fly green needles (NAV mode).  Also; as I was fixating on the VPI during the arrival phase; the VPI dropped out and gave me no vertical information.  This was most likely from the transition from the arrival descent path to the new approach path.  This simply made me lose situational awareness and instead of trying to cross check my point on the FMS and the charted altitudes on my 26L ILS chart I just waited for the VPI to come back.  By the point the VPI returned; I was already descending below 7000 feet MSL about 4 miles before I had reached MKAYE.  Almost simultaneously; ATC had advised me that I needed to be at 7000 before MKAYE and the VPI had recalculated and shown up on the screen; which clearly showed me way too low for the approach.I had realized my mistake and that I was below the charted 7000 feet at MKAYE and began to make a shallow climb to 7000 feet.  That was the only time ATC had mentioned anything about the deviation.The threat was conducting an arrival and approach transition using just the FMS which I was unfamiliar with.  The largest threat I believe was having the VPI drop out when I was using that as my sole source of vertical information.The error was descending below the charted altitude of 7000 feet while using the VPI as my sole source of information.  Also; presetting 2000 feet when I was already task saturated because of the VPI dropping out could be something that posed a possible error in this situation. I would clearly never rely on the VPI as a 100% reliable source of vertical navigation.  Having an understanding of how it works and where it derives its information is essential when planning descents.  Being aware that it will recalculate a new descent path will make me more cognizant of what altitudes and speeds to expect for the remainder of the arrival or approach.  I will always back the VPI up with raw data from charts and mental math.  Also; when work load gets heavy in unfamiliar situations like this it could have been better to set step down altitudes in this approach.  While this may have added to the workload it would have prevented this error. I believe the company just needs to continue to emphasize that while the VPI can be used as your primary source for descent reference; it is essential to always back it up with something else.
EMB145 First Officer reports descending below a step-down fix when transitioning from the DOOBI2 arrival to the ILS26L Approach into IAH. The Vertical Path Indicator was being used to comply with crossing restrictions but it dropped out crossing BOZZZ but the descent was continued until it reappeared and showed below profile. ATC took notice.