Citation Excel (C560XL)
Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor; Flight Crew Multiengine; Flight Crew Instrument
Flight Crew Last 90 Days 100; Flight Crew Total 8900; Flight Crew Type 1800
Human-Machine Interface; Situational Awareness; Time Pressure; Training / Qualification
Deviation - Altitude Undershoot; Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Human Factors; Procedure; Chart Or Publication
We were cleared to descend via the EAGUL 5 RNAV STAR into PHX. The arrival was loaded into the FMS and the aircraft proceeded to descend per the vertical profile. The arrival called for us to cross HOMRR intersection at or below 16;000 MSL then cross VNNOM between 10;000 and 11;000 MSL. The VNAV mode disconnected after we crossed HOMRR at 16;000. We reacted immediately but the required descent from 16;000 to 11;000 MSL must be completed in 4 NM. This is a very steep descent gradient which we were not able to achieve. We crossed VNNOM at 12;000 MSL rather than the required [between 10 and] 11;000 MSL. I feel this crossing needs to be examined and that such a steep descent gradient should be removed for safety.
The reporter advised he seldom flies into PHX and wasn't familiar with the STARS. He stated they had 'manually' flown the descent to cross HOMRR at 16;000 as a fuel saving measure; but when they then engaged VNAV the FMS; which he understands will not exceed either a '3% or three degree' (he was unsure of which) vertical descent path; rejected the VNAV mode at which time they discovered their problem with respect to the crossing at VNNOM; four miles ahead and a minimum of 5;000 FT below their altitude. Reporter readily admitted they had not done due diligence with respect to briefing the entire procedure in a timely fashion.
A C560XL flight crew was unable to comply with the crossing restriction at VNNOM Intersection when cleared to descend via the EAGUL RNAV STAR. The reporter believes the procedure requires too steep a descent between the previous waypoint; HOMRR; and VNNOM requiring; potentially; as much as a 5;000 FT descent in only 4.1 NM.