FLEX Temp — Reduced Thrust Takeoff

Step-by-step: FLEX Temp — Reduced Thrust Takeoff

1. Set the route. Type LOWW/EDDM (Vienna to Munich) and press 1R.

   A short hop on a long enough runway — perfect for FLEX. The lower the takeoff weight relative to runway length, the more thrust reduction you can take.

2. Open the PERF page to access the FLEX TEMP field.

   FLEX is set on the TAKEOFF section of PERF. The FMS uses it to compute reduced N1 (engine fan speed). The actual outside temperature on the ground does not change — only what the engines "think" it is.

3. Try FLEX 40 first — a small thrust reduction near the lower end of what the FMGS will accept. Type 40 and press 4R.

   FLEX 40°C gives only a small thrust reduction. The FMGS rejects FLEX values below TREF (the engine's flat-rated corner — around ISA+15°C, which is 30°C at sea level on a standard day for the CFM56-5B) and below current OAT — so very low FLEX values often won't take. Real airlines rarely flex this little anyway; the engine-wear saving is tiny.

4. Now try FLEX 50 — the typical airline value on a warm day. Type 50 and press 4R.

   FLEX 50°C is a common day-to-day value across European/US short-haul ops. Around 80-90% N1 instead of 100%. Engines run cooler, last longer, and you still have enough margin to reach V2 well before the runway end.

5. Now try FLEX 70 — pushing toward the maximum allowed. Type 70 and press 4R.

   FLEX 70°C is at or near the maximum the FMS will accept (typical Airbus FCOM limit ~70°C absolute, but the real cap is the assumed-temp performance calculation — you cannot flex more than the runway/weather allows). Safe on a long runway with a light aircraft. Forbidden on contaminated runways or near limit weight.

6. Set FLEX back to 50 — the realistic operational value. Type 50 and press 4R.

   Real flights use whatever FLEX value the EFB performance calculation produces — rarely a round number. You enter exactly what dispatch hands you. FLEX 50 here is a stand-in for "the value computed for today's conditions".