Date: 202101
Local time of day: 1201-1800
Location: D01.TRACON, CO
Flight conditions: VMC
Weather elements / visibility: 10
Light: Daylight
Aircraft: Small Aircraft
Operator: Personal
Operating under: Part 91
Flight plan: None
Mission: Training
Flight phase: Final Approach
Airspace: Class E L30
ATC / advisory: TRACON L30
Second aircraft: Commercial Fixed Wing
Reporter function: Pilot Not Flying; Instructor
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Flight Instructor; Flight Crew Commercial
Human factors: Situational Awareness
Anomaly: Conflict Airborne Conflict; Deviation - Altitude Undershoot; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance
Contributing factors: Airspace Structure; Human Factors; Procedure
Primary problem: Ambiguous

Narrative: During a practice RNAV (GPS) [Runway] 35 approach to Colorado Air and Space Port (CFO); we had just crossed the Final Approach Fix (FAF) at ARRES at the charted altitude of 7;500 feet MSL. The student at the controls was 2-3 seconds late in beginning the glideslope intercept; and our glideslope indicated slightly high. Knowing that we had to descend 500 feet in order to be below the upcoming 7;000 feet MSL Class B shelf; I reduced power to increase our rate of descent. There was also an air carrier [Aircraft Y] at our 1 o'clock; approximately 1;000 feet above our altitude according to ADS-B; which was making a right turn from a southerly heading to a northwesterly as it make its approach to DEN. Denver Approach contacted us saying; 'I need you below the Bravo shelf at 7;000 feet.' I replied 'roger; we are increasing our rate of descent; currently indicating 6;800 feet.' The Aircraft Y crossed above and ahead of us; right to left; approximately 1;000 feet above and 200 feet ahead. I did not hear any of the communications between the Aircraft Y and Denver Approach; nor did I notice any maneuvering by the Aircraft Y indicating it was reacting to our presence. The complex airspace around DEN is extremely challenging; and this approach's FAF is approximately 1.4 NM from the 7;000 feet MSL Class B shelf. Descending 500 feet in less than 1.4 NM at small aircraft approach speeds does not require excessive rates of descent; but it does require a timely descent. Any delay; or crossing the FAF above the 7;500 feet MSL altitude; quickly makes it likely that an airplane may violate Class B airspace during the Final Approach Segment. In our case; the timing of the arriving Aircraft Y was coincidentally exactly the same time we were at this critical place. As a CFI-I; I am reluctant to use this approach for training ever again. One suggestion may be to cross ARRES at an altitude lower than the charted minimum of 7;500 feet MSL - if; and only if; it's a practice approach under VFR. The potential negative training this would cause would have to be weighed against the extra margin of separation from the Class B shelf. Additionally; in the future; if I use this approach for practice again; I will have the student execute a missed approach and turn away from the 7;000 feet MSL Class B shelf if the aircraft is high at ARRES; does not initiate a timely descent; or if I notice DEN traffic that could cross above us as happened on this flight.




Synopsis: Small aircraft instructor reported experiencing an airborne conflict with a commercial jet on an approach to Denver airport.