B727 Undifferentiated or Other Model
Flight Crew Commercial
Flight Crew Last 90 Days 150; Flight Crew Total 12000; Flight Crew Type 8500
Human-Machine Interface; Situational Awareness; Time Pressure
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Aircraft; Human Factors
Upon initial approach; approximately 25 NM out; we were assigned to descend to and maintain 2;600 FT MSL. Expect vectors for the ILS approach. Both Captain and First Officer briefed the approach; dialed [the frequency] for the glideslope/localizer. Immediately; both the Captain and First Officer had the glideslope localizer flagged as non operational. We then tried to re-tune in the ILS and did not get the ident (not receiving signal). As soon as this happened we contacted Approach to inquire if the ILS was functional. As per Approach; the facility was functional. We then requested the RNAV/GPS approach. As soon as Approach assigned us the approach; we then attempted to input the approach into our GPS/FMS computer. During this time; we unintentionally drifted below our assigned altitude of 3;000 FT MSL to 2;100 FT MSL. As soon as our altitude warning horn sounded off; an immediate climb to 2;600 FT MSL was initiated. Approach notified us of our altitude. Upon arrival; I contacted Approach on the telephone to discuss the error. The incident occurred during the quick change for the approach from ILS to RNAV approach mode. Before the approach was selected in the aircraft navigational system and entered; the drift below altitude occurred. During this phase of flight the aircraft was on autopilot. However; altitude capture was unavailable for the type of navigational system installed in this particular aircraft. The Director of Operations for the company was immediately briefed on the situation and a follow up report was provided to the company the following day.
B727 First Officer reports losing 500 FT while distracted by an inoperative ILS and the switch to an RNAV approach. A low altitude warning from ATC and EGPWS alert the crew and they return to the cleared altitude. The autopilot was not capable of maintaining an altitude during the switch to the RNAV approach.