First officer reported flight missed an altitude level off because they were a little rusty.

Attributes
ACN	1763430
Time
Date	202009
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Falcon 2000
Operating Under FAR Part	Part 91
Flight Phase	Initial Approach
Flight Plan	IFR
Person 1
Function	Pilot Not Flying First Officer
Qualification	Flight Crew Instrument Flight Crew Multiengine Flight Crew Flight Instructor Flight Crew Air Transport Pilot (ATP)
Person 2
Function	Captain Pilot Flying
Qualification	Flight Crew Multiengine Flight Crew Instrument Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Published Material / Policy Deviation - Procedural Clearance

Narrative:

We were flying to ZZZ; assigned the xx lda starting at ZZZ1 VOR. We were instructed to cross ZZZ1 VOR at or above 15;000 and cleared for the approach. We had programmed the avionics to fly the approach and descend on the approach to the glide slope intercept altitude. During the setup we neglected to arm the VNAV. After passing ZZZ1; descending; the aircraft did not level off at 14;000. We noticed the missed level off just before the controller alerted us to a probable altitude deviation. The captain immediately disconnected the autopilot and returned to the appropriate altitude. The flight concluded uneventfully.I believe there were several factors that contributed to the event. Primarily; the flight crew was legally current but a little out of practice. A full procedure instrument approach in real-time in mountainous terrain is different than currency and maintenance flights in non mountainous terrain. We have a very strong safety culture; attend recurrent training more frequently than required; and have extensive backgrounds including airline and corporate jet experience but are still susceptible to being 'rusty'.

Original NASA ASRS Text

Title: First Officer reported flight missed an altitude level off because they were a little rusty.

Narrative: We were flying to ZZZ; assigned the XX LDA starting at ZZZ1 VOR. We were instructed to cross ZZZ1 VOR at or above 15;000 and cleared for the approach. We had programmed the avionics to fly the approach and descend on the approach to the glide slope intercept altitude. During the setup we neglected to arm the VNAV. After passing ZZZ1; descending; the aircraft did not level off at 14;000. We noticed the missed level off just before the controller alerted us to a probable altitude deviation. The Captain immediately disconnected the autopilot and returned to the appropriate altitude. The flight concluded uneventfully.I believe there were several factors that contributed to the event. Primarily; the flight crew was legally current but a little out of practice. A full procedure instrument approach in real-time in mountainous terrain is different than currency and maintenance flights in non mountainous terrain. We have a very strong safety culture; attend recurrent training more frequently than required; and have extensive backgrounds including airline and corporate jet experience but are still susceptible to being 'rusty'.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.