Issue Six - 2001

Captain In Command (2)Windshear Encounter

Introduction

We are told that the probability ofencountering a microburst wind shearis once in two lifetimes. However theconsequences of an encounter arepotentially so severe that all flightcrews should be trained in the recognition of wind shear and, for aninadvertent encounter, the appropriateflight manoeuvres to escape the hazard. Many modern aircraft haveautomatic detection systems (ATP,BAe146) and some have automaticrecovery flight guidance (Avro RJ).

The following report is of an incidentinitiated by a wind shear encounter; itis an excellent example of many goodaspects of flight safety, in particularhuman behavior - experience,

training, CRM and decision making.

Capt. Ray Sherwood and F/O. CraigRable were crewing an Air ZimbabweBAe146 - Flight UM 225 fromVictoria Falls to Hwange. Ray was inhis last two weeks of his aviationcareer prior to retirement; he wouldhave hoped not to encounter windshear nor have to deal with the ensuing diversion with aircraft systems failures and a landing withunsafe gear warnings. But he did.Furthermore, Ray , Chris, the otherfive crew members and all fifty ninepassengers survived to tell the tale.For all of us this incident is a classicfor flight safety and a first classexample of a 'Captain in Command' ( See JETSETS Issue 5)

Lesson. Any probability of encounter-ing a hazardous situation does notrecognise age, experience or lengthof service - if it can happen it willhappen, probably to you!

Narrative Capt. Ray Sherwood

The set up.

"We were approximately 21nm fromthe airfield. We had the runway insight during the descent once wewere below the general cloud (FL105 7000ft a.g.l.). No Cumulonimbuswas observed, but there was somerain falling from a large Cumulusnear the airfield. The weather radarwas on and showed weather returnson track. ATC were asked for furtherweather information. The reply was

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'light rain was falling at the airfield'.The runway in use was 08, wind020/10kts, QNH 1013, OAT +32ºC,Few at 7000ft, Visibility 10km plus.(A nice summers day in Africa)

I realised that the rain area wouldprobably be on our final approach.Thus the briefing was for the NonPrecision VOR NDB 08 (see chart).As a supplement to the approachplate I noted several passing altitudesfor DME ranges - the DME was not adeclared nav aid but was in service -the altitude and distances were a backup. At 'PO' the altitude should be4700ft and the DME reads 3.8 nm.The Minima at 'WN/VWN' is 3850ft(307ft a.g.l.) this requires an 850ftdescent in 3.8 nm i.e. approx. 220ft/nm. The radio altimeter bug wasset to 310ft for advisory purposesonly. F/O Rabie was briefed to callout the DME range and altitudes. AirZimbabwe's standard operating procedure requires that at 500ft aglthe aircraft should be in the slot. Theslot is :-IAS Vref +5 to +15 ktsRate of descent 600 to 800'/min ILSLOC and G/S within 1 dot . ADF 5deg L or R of inbound track. If thesecriteria are not met a go around is tobe carried out. - On this approach wewere about to reach these decisionpoints!"

Lesson. Always prepare back upplans:- use all available aids, worktogether as a crew, use an instrumentapproach when VFR flight is indoubt; note altitudes with distance,calculate the descent path - thesepreparations may not always be usedbut when things go wrong��.

And things do go wrong.

"The aircraft crossed 'PO' in the landing configuration at 4700ft.DME 3.8 - with all checks completed. The pitch attitude on anapproach with normal approachpower of 65% N1 is 3 degrees nose

down. After 'PO' we flew into rainwhich increased in intensity as wegot closer to the runway. The noiselevel of the rain hitting the aircraftbecame louder. The turbulenceincreased; the instruments were allperfectly visible i.e. the turbulencewas not severe."Just before the 'Standard' 500ft call -we were approaching 4050ft (QNH) -I heard the F/O Say 'What the.�..'

At 4050ft the aircraft suddenly andvery rapidly descended towards treetop level (the actual altitude was notobserved on the radio altimeter).The stick shaker operated. I immediately advanced the thrustlevers, checked the pitch attitude,which was already 4 degrees noseup.The IAS had dropped to Vref minus

l0 kts.Full power was applied.The pitch attitude was held and thenadjusted more nose up.The stick shaker was respected as perinadvertent wind shear encounter drillby adjusting the pitch attitude.F/O Rabie called 100ft to minima(3850ft) and recalls the altimetergoing down very fast. I concentratedon flying a suitable pitch attitude andheading. A second stick shaker activation occurred. The pitch attitudeat onset of both stick shakes was wellbelow the normal go around attitude.The aircraft attitude was againincreased avoiding stick shaker warn-ings. Target attitude was 12 degreesnose up.

Lesson. Remember what was taughtin the simulator - wind shear go-

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around - maximum power, aim for apitch attitude of 12 degrees butalways respect the stick shaker.

The aircraft climbed away through some treetops in the climbout! i.e. from below the height of thetrees. The airfield chart shows treeson the 08 end at 3599ft. The 08threshold elevation is 3543ft. Thiswould give a possible tree top height(12 years ago) of 56ft. Some growthcould have taken place of course.

The flaps and gear were only retracted when clear of possibleground contact, at this stage I wantedto have the best aircraft performancein case we had engine failures due todamage sustained.In the initial climb away we were outof the rain and in clear weather.The aircraft was levelled off at 6000'and 180kts in a clean configuration.The engines when checked werefound to be all operating normallywith no abnormal vibration levels. Iobserved the airfield ahead. The

visibility from 'WN' to the airfieldwas more than l0km.with no cloudup to 7000ft"

--------------------------

At this time Ray and his crew hadsurvived what has been subsequentlyshown to be a very severe microburstwind shear (once in two life times?)so what next. Just imagine what thecrews the state of mind was - whatwould you do next? Deal with thenext level of emergency?

Lesson. If you have been unfortunateto have a severe safety event you arenot exempt from another! Havingone event can to lead to a consequen-tial event.

What next?

Clearly the aircraft was damaged; -double hydraulic failure!The Green hydraulic system 'LoQuantity' and Yellow hydraulic

system 'Hi Temp' Abnormal check-lists were carried out (No flaps,emergency gear lowering, no ruddersno spoilers/lift dump). An evaluationof the aircraft state was made withvisual inspections of the engines,landing gear, flaps and wings beingcarried out from the cabin by an onboard engineer. He reported thatthere was no visible damage or leaksevident. After crew consultation andagreement the decision was to divertto Harare. Note: Hwange is a remoteairfield; the nearest medical facilitiesare 75 nm distant.The Senior Flight Attendant wasinformed; I informed the passengersusing the PA as to what had hap-pened and our intention to divert toHarare. ATC were informed and clearance obtained to climb to FL 90.This kept us clear of cloud. Later weclimbed to FL 110 but the pressurisa-tion was not used due to concern over possible structural dam-age. The IAS was held at 200ktswithin the Gear limiting speed of205kts. The Fuel remaining was

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Fig 1

now going to land; "ATTENTIONCREW ON STATIONS" call wasmade when the flap was selected to33 the Gear warning horn operatedbecause the gear was not lockeddown. 30 flap was reselected for ashort time. 33 flap was reset at about800ft.

The approach was flown with thegear indications: Left GREEN, NoseRED and Right REDThe F/O made the "BRACE" call onPA at 250ft.I made the landing on the Left mainwheel only, this was to keep directional control and also ourcrosswind technique. The Right mainwheel was gently lowered onto theground. The Right main gear heldand in fact was down and locked. Atthis time the nose was held up clearof the runway; at approximately80kts and as elevator control wasdecreasing I gently lowered the noseonto the runway. The ground contactwas "juddery" and the noise levelincreased.The Yellow wheel brakes wereapplied and the Aircraft brought to a

5100kg. The fuel flowwas 400kg/hour perengine.

Lesson. Evaluate thesituation, decide on theappropriate course ofaction. Use all availableresources; then commu-nicate - this is an essential part of CRM.

The cabin crew carriedout a full EmergencyBriefing en route. (Thetour leader translatinginto French solved theproblem of giving thenon English speakingFrench tour group theemergency briefing).The PA was used tokeep cabin staff andpassengers informedduring the flight. Harareairport was informed ofour status and warned toexpect a fly past if wehad landing gear prob-lems. The weather atHarare, which wasgiven as fair had now changed tolarge rain showers. Final preparationswere made in the holding pattern.

When cleared for a fly past of thetower to check the landing gear theYellow Hydraulic system was re-instated, it operated normally andfor the rest of the flight. 18 flap waslowered, the gear was lowered usingthe emergency system. As had beenexpected we had a problem. The gearindications were Left GREEN, NoseRED, and Right RED The fly pastwas made at 160kts to the left of thetower, ATC reported that they wereunable to ascertain the exact status ofthe gear. However two light aircraftpilots taxiing came on the radio tosay that our main gear was down butthe nose gear was up. At this stagewe concentrated on getting the rightmain gear down. The aircraft wasyawed to see if the gear doors woulddrag the gear fully down. The engineer in the cabin reported that itappeared to be down but there wassome damage visible to the geardoors. We now increased the IAS to

190 kts and the aircraft was thenpulled up to see if "G" forces wouldget to the nose gear down. It stayedUP. Whilst turning the Right maingear GREEN light came on but laterit was to revert back to RED

Lesson. Continue to evaluate the situation, take time if available, donot rush.

Emergency landing

A further tower fly by was plannedbut due to heavy rain and turbulenceon the approach it was abandoned. Itwas now almost dark. The Fuel wasdown to 1200kgs and havingexhausted all means of getting thegear down we had to land. The landing weather was:- 330/08 kts,light rain falling and the visibilitygood.The R/W had been foamed for theNose gear UP landing. ATC were nottoo sure of its effectiveness as theheavy rain might wash the foamaway.The Cabin was told that we were

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had minor injuries which were sus-tained during the slide evacuations.

Lesson. A text book example of anevacuation - checks, CRM and communication.

FDR and incident analysis.

The Zimbabwe authorities in accordance with ICAO Annex 13investigated the accident. BAE SYSTEMS was given access to theFDR and requested to assess the aircraft flight path and windshearconditions.Fig 1 shows the FDR radio altitudeplotted against along track distancebased on the computed ground speed;the lowest altitude was 35 ft. Byevaluating the FDR airspeed and attitude data in a design simulator itwas possible to reproduce the aircraftactual air-mass flight path. In Fig.2the horizontal profile, headwindshear, is shown as a solid line,increasing headwind being positive.As a comparison the dashed lineshows the horizontal wind for theFAA 'Level 4' wind shear trainingmodel. Fig 3 shows the correspond-ing vertical velocity, down burst; theFAA profile is again shown as a

stop. As we were stopping I shut offthe fuel levers and once stoppedapplied the parking brake. On the PAI ordered the "EVACUATION" withboth sides of the aircraft to be used.

The evacuation

The F/O carried out his Evacuationdrills which I monitored I gave theF/O the fire extinguisher; I took mytorch. I estimate an elapsed timeabout 50 secs since ordering theEvacuation. I checked the wholeCabin and Toilets for people nonewere found. The Emergency lightsystem was working and giving goodlighting in the cabin. As I was aboutto exit the Aircraft the F/O asked meto switch off the aircraft batteries.Once out of the aircraft I checked onthe passenger evacuation. All hadbeen accounted for; 39 on the righthand side and 20 on the left. Thecabin crew was also OK. I made myway to the right hand group of pas-sengers and cabin crew. Once we haddone all that we could at the Aircraftthe F/O and I were taken back to theairport by car. We joined the passen-gers and Cabin Crew in the Businessclass departure lounge. Here the AirForce Doctors gave us a medicalcheck. Of the 59 Pax I understand 10

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dotted line. A windshear downburst isshown a negative value.

The simulation showed that the aircraft encountered a classicmicroburst wind shear. The core ofthe down flow appears to have beenapproximately 7,000 ft (1.6 nm)diameter. The overall microburstdiameter, including out flow and updraught is approximately 20,000 ft(3.3 nm).

The aircraft's initial, undetected,encounter with the microburst waswith the increasing head wind and updraught. However as the head windand up draught changed to a severedown draught, the aircraft's verticalflight path followed the vertical windprofile. The head wind sheared rapidly, -40 ft/sec to zero, a decreaseof 25 kts, at the same time as encoun-tering the peak down draught of -30ft/sec, a descent rate of 1800 ft/min.The rapid loss of airspeed resultedfrom the head wind shear. The initialstall warning occurring in the downdraught, probably due to changes inangle of attack rate as opposed to thesteady angle of stall a warning.

Fig 1

Fig 2

Issue Six - 2001

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The elapsed time fromthe initial crew awareness at 880 ft tothe minimum altitude of35 ft was only 20 seconds.

Thus the aircraft's average verticalspeed was 2400 ft/min; approximate-ly a 12 degree flight path.

During the manufacturers certifica-tion stall tests and simulator tests ofstall recovery in the landing configuration with approach powersettings of 60 to 65% N, the stickshaker operated at pitch attitudes of10 to 12 degrees nose up. The IASfor stick shaker operation at theLanding weight of 33,356kgs was97kts. The IAS at the stick shake inthe windshear (104kts ) was wellabove the normal stall speed of97kts; the pitch attitude was only 4degrees nose up, this was probablydue to the turbulence within themicrobust. Refer to the BAe146MOM Vol 2 write up on windshear.

The crew actions were in accordancewith Manufacturers OperatingManual (MOM) and the aircraft performance achieved was as anticipated for the conditions.

The BAe146 characteristics of having high lift, rapid engineresponse and robust constructionmade a positive contribution to thisincident. If the microburst encounterhappened at any lower altitude or ifthere was any lesser crew response,then the resulting aircraft flight pathcould have led to a major accident.

Lesson. Follow recommended procedures to give the optimum performance.Practice an encounter with the FAAlevel 4 wind shear in the simulator.

Lesson. Severe wind shear or microdownburst conditions can occur nearany cumulus clouds; they are not limited to Cb clouds.

Lesson. The most significant yetunusual warning of the encounterwas the crew's awareness of theexceptional noise of very intenserain.

Epilogue

Captain Ray Sherwood has nowretired from active flight duty, wewish him a safe and lengthy retirement. However many of youmay meet him during occasions oftemporary duty in the BAe 146 simulators; take heed of his advice

BAe 146 and ATPService Bulletins install a

windshear warning system that provide basic

flight directorguidance for recovery.

�Enhanced� GPWS provides a windshear

warning system.

and experience.First Officer Craig Rable continuesflying with Air Zimbabwe. He isprobably their most experienced pilotin windshear (if not the most experienced in the world).

Lesson. Experience cannot betaught; it can only be gained. Toavoid gaining unnecessary windshear experience, note the abovelessons in Captaincy, approachpreparation, flight procedure, CRMand decision making.

Lesson. Windshear, like many othersevere aviation weather hazards, isbest avoided. Note the warning signs- clouds, temperature, and rain - if inany doubt during an approach, commence a go around immediately;if on the ground do not take off.

Fig 3