SPEED DECAY AT CRUISE th
18 AIRBUS SAFETY CONFERENCE - BERLIN
A330 SPEED DECAY EVENT
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BOM to AMS FL380 over NW IRAN
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Clear skies at dawn with a thin undercast
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Airspeed 260 to 197 knots gradually
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Autothrust ON / Autopilot maintained altitude
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Maximum available thrust did not stop decay
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Pilots received no warnings or messages
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Pilots took over and flew manually
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Descent to FL 340 and continued to AMS
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CREW INFORMATION •
Captain on rest break
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Right seat FO (Pilot Flying)
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Left seat FO Pilot In Command (Pilot Monitoring)
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Crew noticed speed at 240 knots
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Autothrust commanded climb thrust
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Speed continued to decrease
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CREW FEEDBACK
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Assessed situation understanding 1. Normal law and flight protections 2. A330 will not stall in Normal law 3. Alpha Protection would limit AOA below stall
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Unsure why speed decay occurred
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RECOVERY
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Crew disconnected AP at 198 knots and descent to FL360 initially, then FL340
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Once they reached normal speed at FL340 the flight was routine
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There was more thrust available at the lower altitude
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UPON ARRIVAL AMS
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Investigation begins
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Maintenance inspects aircraft
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Flight data sent to Airbus and P&W
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Performance Engineering, Tech OPS, Safety analyze FOQA data
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Maintenance inspection results in no engine or aircraft malfunction
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Potential atmospheric anomaly
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ATMOSPHERIC CONDITIONS
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Proper altitude for weight and OAT
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At FL380 for 21 minutes
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Temperature increased slowly about 3 degrees (tropopause entry)
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Horizontal wind change •
Performance increasing wind created a high speed and low thrust state
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Decreasing performance wind slowed the aircraft
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FLIGHT DATA Blank
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THERE MUST BE MORE TO IT…
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The horizontal wind was manageable
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Autothrust soft mode may have contributed, but was not the cause
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There was some indication of a vertical wind component
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When pitch exceeds AOA during level flight this indicates a downdraft
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TOULOUSE MEETING •
Airbus calculated the vertical wind with performance modeling
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It averaged 1000 fpm over four minutes
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Why did it last so long
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Delta meteorology theory
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Perpendicular gravity wave encounter in the tropopause
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GRAVITY WAVE THEORY •
Gravity waves are generated in the troposphere by frontal systems or by airflow over mountains
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The amplitude increases in the tropopause
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Picture a surfer under a curl for a sustained period due to a perpendicular direction of travel vs. the wave
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SIGNIFICANT VERTICAL WIND •
Current weather forecasting tools cannot model or predict
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New FOQA algorithm for vertical wind •
Pitch > AOA in level flight = downdraft
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FOQA study shows this vertical wind occurs on this route over Turkey, Iran, and Pakistan in the winter/spring months
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Winter/Spring months have higher seasonal winds Winter/Spring trop heights are lower than Summer/Fall
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WAVE CLOUDS
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LIKE EVENT STUDY •
All fleets checked for below Vls speed at cruise • •
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About a dozen flights had lost significant airspeed and some had to descend •
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1.4 million flights This was only event
AT soft speed mode saves 1% on fuel, but will cause a slight delay (up to 30 seconds) in reaching full CLB thrust on A330
Checking 5 years of A330 flights showed about 2 per year went below green dot at cruise altitude •
Most of these where in moderate or greater turbulence
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Not as insidious as our subject event
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AMS-BOM-AMS 39 Events (Nov-May)
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CONCLUSIONS •
This rare downdraft exceeded aircraft performance (All fleets) • Perpendicular direction • High winds over terrain • Tropopause height
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No low speed warning exists at high altitude
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High altitude speed awareness is key • Severe downdrafts • Windshear • Airspeed dropouts
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NEXT STEPS •
Support industry change for a low speed warning on new aircraft •
Current A330 design has a low energy warning below 2,000ft AGL
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Assist meteorology to develop a model to predict and avoid gravity wave
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Pilot training to manage this rare threat
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DOWNDRAFT STUDY RESULTS •
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Most were over IRAN • 80 knots plus wind • Upwind high terrain • Only NOV-MAY over 3 year look back Why only NOV-MAY? • Other times of year the tropopause altitude is above our normal cruising altitudes • Winter jet streams average twice the velocity of summer jet streams Avoidance strategies • Flying below the tropopause, stay below the wave • FL340 max altitude will provide extra climb performance Other events near mountainous regions and even oceanic areas
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PILOT EDUCATION •
Rare Upper Air Downdrafts can cause; Insidious speed decay even with full thrust applied The Autopilot simply trades airspeed for altitude This downdraft lasted 4 minutes Below green dot drag increases faster than lift Descent to recover airspeed is only option High thrust descent preferred - Not OPEN Descent •
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Proper recognition and recovery is key It all starts with monitoring your speed at cruise
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QUESTIONS?
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