Boeing 737 Memory items – Airspeed Unreliable

JohanAero

When something goes wrong while flying on the Boeing 737, we use the QRH (Quick Reference Handbook). We just have to find out the correct NNC (Non Normal Checklist) and then we just read it and execute it.

For some of these checklists, we have memory items, meaning we have to know by heart some actions.

I have listed 9 memory items:

Airspeed Unreliable

Autopilot (if engaged).......................................Disengage
Autothrottle (if engaged)....................................Disengage
F/D switches (both)................................................OFF

Set the following gear up pitch attitude and thrust:
     Flaps extended.................................... 10° and 80% N1
     Flaps up........................................... 4° and 75% N1

The main point with airspeed unreliable is to identify it quickly. Failure to do so could lead to an aircraft upset. Unreliable airspeed indications will most probably result from blocking or freezing of the pitot/static system or a severely damaged or missing radome.

One or more of the following may be evidence of unreliable airspeed or mach indications:

  • Speed/altitude information not consistent with pitch attitude and thrust setting
  • SPD failure flag
  • SPD LIM failure flag
  • IAS DISAGREE alert
  • Blank or fluctuating airspeed display
  • Variation between captain and first officer airspeed displays
  • Radome damage or loss
  • Overspeed warning
  • Simultaneous overspeed and stall warnings

As soon as we get one of these, we need to think “Airspeed unreliable” and do the memory items quickly to maintain the aircraft stable. Then we will read the QRH which will guide us to the PI (Performance Inflight) tables. Thanks to these tables, we will be able to land at an airport using the correct pitch attitude and thrust for the corresponding phase of flight.

It is also important to remember that the following are reliable:

  • Attitude
  • N1
  • Ground speed (Many air traffic control radars can also measure GS)
  • Radio altitude

Last but not least, STICK SHAKER, OVERSPEED WARNING and AIRSPEED LOW alerts may sound erroneously or simultaneously.

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