What was found
AT FL400 CABIN ALT HIGH WARNING DONNED MASKS. RAN QRH. INITIATED EMERGENCY DESCENT. CABIN ALTITUDE RATE REACHED 2000 FT PER MIN DOWN AND HIGHEST ALTITUDE REACHED WAS 15 K. EMERGENCY DECLARED AMM: 21-31-02-4, AMM: 21-31-00-5, AMM: 05-51-24-2, CABIN PRESS LOSS OCCURED DUE TO LOSS OF L ENG BLEED SOURCE (DUE TO N3 TACH SIGNAL LOSS WRI# 36-11/1704) SINGLE PACK OPS NOT AUTHERIZED ABOVE 35K. ALL ALERTS CONSISTANT WITH THIS CASE. R & R CABIN PRESS CONTROLLER 2. OPS CK OK. ALSO TESTED CABIN PRESS CONT 1 & MANUAL CABIN PRESS CONT. ALL SYS OK. AS PRECAUTION PERFORMED CABIN PRESS LEAKAGE TEST.(DECAY CK) DECAY TIME IN NORM LIMITS. NO LEAKS NOTED. PACK 1 & 2 OPS NORMAL. ENG BLEED OPS CK'D UNDER WRI# 36-11/1704.