Date: 201504
Location: ZZZZ.Airport, FO
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Takeoff / Launch
ATC / advisory: Tower ZZZZ
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Confusion
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Weight And Balance
Detector: Person Flight Crew
When detected: Taxi; Routine Inspection
Result: General None Reported / Taken
Contributing factors: Aircraft; Manuals; Procedure; Weather
Primary problem: Ambiguous

Narrative: Several months ago I flew an airplane with a retractable landing light stuck in the extended position.  The 737 MEL reference for the placard/deferral was 33-XX.  That reference states that there is a 1;500 pound enroute penalty for any light stuck out; PLUS a reduction of 400 pound per EACH LIGHT extended to be applied to Climb Limited Takeoff Weight; Runway Limited Takeoff Weight; Climb Limited Landing Weight; and Runway Limited Landing Weight.  Let's focus on the approach and landing phases.  Recently; we were taking off where the combination of elevation and temperature frequently requires us to limit our takeoff weights.  We pushed back from the gate; got our Load Closeout; and started taxiing for takeoff.  During our Takeoff briefing; I had commented that we were right up against our takeoff weight; and the Closeout confirmed that we were 500 pounds ABOVE our maximum Takeoff weight. We had to wait to takeoff until we had burned off enough fuel to bring us within limits.  As we took the runway for takeoff; it was pretty dark; so I reached up to turn on the retractable landing lights; but stopped as I remembered the recent experience I had with the MEL and retractable lights.  Was I just about to put myself 800 pounds OVER?!  If there is a penalty of 400 pound per light for each light that is stuck in the extended position; and the 737 does not 'auto-retract' in the event of an engine failure; I believe there should be verbalization/analysis of that 800 pounds; if the retractable landing lights are to be used.The same applies quite often where the combination of altitude and temperature can put you close to your Climb Limited Landing Weight. How often do we land there at night close to the limit; yet nothing has ever been mentioned (as far as I know) about this potential problem? In the [first] scenario; if I were to lose an engine during a heavyweight takeoff; the last thing on my mind would be 'Gee; better retract those landing lights!' When I land there at night; I want every light I have blazing; believe me; but not if it puts me over the Climb Limited Landing weight.I'm honestly not sure.  If it truly isn't a problem; then why does the MEL proscribe a penalty when they are stuck out?  If there is a penalty when they are stuck out; shouldn't we be accounting for the weight differences every time we plan on using these lights for takeoff and landing?  I ride the jumpseat between home and work frequently; and see many different 'techniques' on landing light usage.  Some guys use them to confirm they have received their landing clearance...others use them when 'cleared for the approach...'  We are all creatures of habit and checklist; and I would hate to reach up and turn them on out of habit; and then bust a limit.




Synopsis: Since the MEL penalty for a landing light stuck in the extended position is 400 pounds per light; a B737 Captain wonders if landing light usage is considered by the weight and balance software for weight limited takeoffs and landings at hot; high altitude airports.