Date: 201508
Local time of day: 1201-1800
Location: DAL.Airport, TX
Flight conditions: VMC
Light: Daylight
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class B DAL
ATC / advisory: TRACON D10
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Human-Machine Interface
Anomaly: Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Unstabilized Approach
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented; Flight Crew Took Evasive Action
Contributing factors: Aircraft; Company Policy; Human Factors
Primary problem: Human Factors

Narrative: I haven't had much experience in the [737-Classic] over the last few months. I realized as soon as we sat down in the airplane that I was making little mistakes; things like arming the LNAV on the ground; forgetting that in the older model aircraft it will disarm at lift off and need to be selected again. It just didn't feel comfortable or familiar.We are on day three of a four-day trip. It was my leg. Approach cleared us direct to the FAF and then subsequently for the visual approach. I had the aircraft navigating direct to the FAF and was using V/S mode to be over the FAF at 1900 ft. Meanwhile; around two miles from the fix; I asked the Captain to lower the landing gear and select flaps 15. My plan was to have LNAV intercept the final approach course; then select VOR/LOC and finally APCH as we crossed the fix. I reached up to select VOR/LOC and nothing happened. I tried APCH and nothing happened. Meanwhile the airspeed continued to decrease. My 'plan' was to have the airplane intercept the glideslope and that would maintain the speed. Unfortunately; the airplane maintained 1900 ft as instructed. By the time I looked back at the airspeed indicator; we were at TARGET but still at flaps 15! I quickly disconnected the automation; lowered the nose; and added quite a bit of thrust while simultaneously asking for flaps 30. About that time; I realized that my switch was still in NAV. I flipped it down to VOR and asked the Captain to arm Approach. The remained of the approach was concluded without incident.This event scared and embarrassed me. It was unacceptable to allow the airplane to get into that energy state especially low to the ground. I take full responsibility for my actions; but I wanted to submit this ASAP to offer the thought that as there are fewer and fewer [737-Classics] in the fleet it should come as no surprise when we don't fly one for several weeks or even months. So when all of a sudden you find yourself in one; it doesn't feel comfortable or familiar. It isn't the lack of screens. I flew round-dial DC9s; 737s; Saabs; and Jetstreams in my previous life. It is the fact that there are subtle differences between the types that are easy to forget about when you haven't flown in a while. My solution would be for our Company to build pairings that are entirely older model or entirely new model aircraft. Don't swap back and forth between types during a single trip. As for me; I'm going to look for some trips with a little more older model flying and reacquaint myself with the type.




Synopsis: B737 First Officer reported mistakes made while flying the B737-Classic after flying only B737-NG aircraft for months.