EMB ERJ 145 ER/LR
Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Aircraft Equipment Problem Critical
Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Aircraft; Human Factors
We were out of 9;000 FT for 8;000 FT when I noticed amber dashes on altitude alerter. We had gone into pitch mode; and we noticed that the First Officer (Pilot Flying) had a red 'X' that appeared on his altimeter. He selected ADC1; and I ran the QRH. The aircraft had gone into heading mode inadvertently as well; and we momentarily headed left of course on our way from MERSR to METRO. We immediately began proceeding to the right to join our course; and as we did; ATC asked if we had navigational issues. I advised them that we had a minor maintenance issue and were once again joining back on course. They accepted; and we joined our proper course to METRO without incident. We had already had Pack 1 MEL'ed on the aircraft; and during flight; we kept getting an RA1 flashing; indicating RA2 had quit.  This was sporadic and would disappear. During our descent on the PHLBO arrival; we got the failure on the First Officer altimeter tape.  Additionally; the weather was marginal; with low ceilings around 500 FT; and visibility 4 miles. Regardless of what mechanical issues present themselves; I must remain constantly aware of the aircraft position. Broken clouds and rain.  We were in and out of clouds.
In flight altimeter 2 failed on descent from 8;000 FT to 6;000 FT. this kicked off the Nav guidance along with the vertical guidance. We put the airplane into heading mode in order to run the QRH and forgot to re engage Nav mode after QRH was complete. I noticed that we were off course and noticed that we were still in heading mode. Started a turn to intercept course to MARRT and ATC queried about what we were doing. We notified them that we had a mechanical issue which caused the deviation and they recleared us direct MARRT to rejoin the arrival. Navigation kicked off was the threat. Not sure why it was kicked off for an issue that was with the altimeter. Just remember what mode the autopilot is set in.
EMB145 flight crew experiences an altimeter failure during descent causing the autopilot to disengage. Heading mode is selected while the QRH is consulted and NAV is not selected until after a track deviation occurs.