20-4-1[speed decay at cruise]

7/31/2019 20-4-1[Speed Decay at Cruise]

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SPEED DECAY AT CRUISE

18thAIRBUS SAFETY CONFERENCE - BERLIN


2/20

DELTA AIR LINES, INC.

A330 SPEED DECAY EVENT

BOM to AMS FL380 over NW IRAN

Clear skies at dawn with a thin undercast

Airspeed 260 to 197 knots gradually

Autothrust ON / Autopilot maintained altitude

Maximum available thrust did not stop decay

Pilots received no warnings or messages

Pilots took over and flew manually

Descent to FL 340 and continued to AMS

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3/20


CREW INFORMATION

Captain on rest break

Right seat FO (Pilot Flying)

Left seat FO Pilot In Command (Pilot Monitoring)

Crew noticed speed at 240 knots

Autothrust commanded climb thrust

Speed continued to decrease

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4/20


CREW FEEDBACK

Assessed situation understanding

1. Normal law and flight protections

2. A330 will not stall in Normal law3. Alpha Protection would limit AOA below stall

Unsure why speed decay occurred

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5/20


RECOVERY

Crew disconnected AP at 198 knots and descent to FL360 initially,then FL340

Once they reached normal speed at FL340 the flight was routine

There was more thrust available at the lower altitude

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6/20


UPON ARRIVAL AMS

Investigation begins

Maintenance inspects aircraft

Flight data sent to Airbus and P&W

Performance Engineering, Tech OPS, Safety analyze FOQA data

Maintenance inspection results in no engine or aircraft malfunction

Potential atmospheric anomaly

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7/20


ATMOSPHERIC CONDITIONS

Proper altitude for weight and OAT

At FL380 for 21 minutes

Temperature increased slowly about 3 degrees (tropopause entry)

Horizontal wind change

Performance increasing wind created a high speed and low thrust state

Decreasing performance wind slowed the aircraft

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8/20

DELTA AIR LINES, INC.8

FLIGHT DATA

Blank


9/20


THERE MUST BE MORE TO IT

The horizontal wind was manageable

Autothrust soft mode may have contributed, but was not the cause

There was some indication of a vertical wind component

When pitch exceeds AOA during level flight this indicates adowndraft

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10/20


TOULOUSE MEETING

Airbus calculated the vertical wind with performance modeling

It averaged 1000 fpm over four minutes

Why did it last so long

Delta meteorology theory

Perpendicular gravity wave encounter in the tropopause

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11/20


GRAVITY WAVE THEORY

Gravity waves are generated in the troposphere by frontal systemsor by airflow over mountains

The amplitude increases in the tropopause

Picture a surfer under a curl for a sustained period due to a

perpendicular direction of travel vs. the wave

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12/20


SIGNIFICANT VERTICAL WIND

Current weather forecasting tools cannot model or predict

New FOQA algorithm for vertical wind

Pitch > AOA in level flight = downdraft

FOQA study shows this vertical wind occurs on this route over

Turkey, Iran, and Pakistan in the winter/spring months

Winter/Spring months have higher seasonal winds

Winter/Spring trop heights are lower than Summer/Fall

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13/20

DELTA AIR LINES, INC.13

WAVE CLOUDS


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LIKE EVENT STUDY

All fleets checked for below Vls speed at cruise

1.4 million flights

This was only event

About a dozen flights had lost significant airspeed and some had todescend

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 wentbelow green dot at cruise altitude

Most of these where in moderate or greater turbulence Not as insidious as our subject event

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15/20


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

No low speed warning exists at high altitude

High altitude speed awareness is key

Severe downdrafts

Windshear

Airspeed dropouts

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17/20


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

Assist meteorology to develop a model to predict and avoid gravitywave

Pilot training to manage this rare threat

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18/20


DOWNDRAFT STUDY RESULTS

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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19/20


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

Proper recognition and recovery is key

It all starts with monitoring your speed at cruise

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QUESTIONS?

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20-4-1[speed decay at cruise]

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