EASA SIB 2015-07: Prevention of Hazardous Low Speed at High Altitude Cruise

What to do

Flight crews should be reminded of the flight physics principles related to thrust and drag at high altitude and trained to recognize the signs of entering the reversed command zone. If speed decreases despite maximum thrust, crews should promptly initiate a controlled descent and coordinate with air traffic control before returning to cruise altitude. There are no mandatory compliance deadlines as this bulletin provides non-mandatory recommendations.

Summary

EASA Safety Information Bulletin 2015-07 titled 'Prevention of Hazardous Low Speed at High Altitude Cruise' addresses the risk of loss of control due to hazardous low speed conditions during high altitude cruise above flight level 300. It applies to all aeroplane type designs capable of cruising above FL 300 and provides guidance to flight crews on managing Mach number to prevent stall situations. The bulletin emphasizes understanding flight physics related to thrust and drag at high altitude to avoid entering unstable speed regimes.

What changed

This new safety information bulletin introduces guidance to complement existing upset recovery training by focusing on prevention of hazardous low speed conditions at high altitude cruise. It explains the aerodynamic phenomenon of the 'back side of the drag curve' and recommends flight crews initiate a controlled descent if maximum thrust cannot maintain speed, to avoid stall. No mandatory airworthiness directives are issued as a result of this bulletin.

Why it matters

This bulletin is important for aviation professionals as it highlights a critical aerodynamic risk that can lead to loss of control incidents at high altitude cruise. Operators and flight crews gain practical knowledge to better manage aircraft speed and thrust limitations, enhancing flight safety. Compliance teams and training organizations can use this information to update pilot training and operational procedures to mitigate stall risks at high altitude.