Medium Transport; Low Wing; 2 Turbojet Eng
Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 85; Flight Crew Total 6600; Flight Crew Type 430
Situational Awareness
Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented
Human Factors; Procedure
Assigned a heading; we were told to expect the Visual Approach to RWY 24 into TTN. Tonight we were being vectored and I noticed we were on a downwind; but we were further south of what would be a standard left traffic pattern. About midfield left downwind; we advised ATC we had the field in sight. After a short period of time we were issue a vector of 010 and cleared the Visual 24. I noticed that a vector of 010 would have us turning base to final at what I estimated to be a 2.5 mile final with the aircraft setup for a 129 degree intercept. We continued to descend and level off at a 1000' in order to fully configure and complete checks. After level off; we received a low altitude alert from TTN Tower. We were approximately 3 NM from the extended centerline of RWY 24. We turned base to final at 1000' white over white on the PAPI. We were stabilized; acquired normal glide path at about 2.0 NM. What occurred was not unsafe. However; a normal traffic pattern which I have flown many times into this same airport would have eliminated any guess work and we wouldn't have received the low altitude alert from Tower. During a normal visual pattern approach; I can discern distance from the runway off the wing and appropriate altitude in relation to the runway visually. An overlay extended centerline is used only to confirm the correct RWY. With a vector to a visual at that kind of intercept angle; while I can see the airport; you're forcing crews to estimate where altitude should be in space by referencing the overlay extended centerline relative to a 3 degree glide path; requiring our heads to be down. The vector we were given tonight required us to turn final less than 3 miles from the runway; requiring us to be fully configured and at approach speed (135KIAS) prior to making the base to final turn. During vector to visual approaches; I recommend that ATC communicate what distance the vector to final will achieve from the runway. Additionally; altitude guidance should be given with vector angles less than 90 degrees from the intended runway.  I interpret 'cleared the visual approach' to mean I am free to maneuver so as to conduct a normal stabilized approach by visual reference to the runway while honoring published approach profiles. That's precisely what we did tonight and we got an altitude alert from TTN Tower. What I can do differently in the future to prevent future occurrence is to use the CAT D published circling altitude if there is an IAP to serve as a better way to plan our descent near the airport. In this instance; I was thinking visual approach during the approach; but I could have very easily drawn a 3.6 NM circle around Runway 24 and used that as a reference as well.  ATC does a terrific job here; this is the first time I have ever had a nonstandard event here and I have been in into TTN dozens of times in [recent] years.
Medium Transport Captain reported receiving a low altitude ATC alert on a visual approach to TTN airport.