Estate Autunno 2000

Thunderstorm Encounters In The Airport Terminal Area

TS = ThunderStorm = Tricky Season

L'estate è la stagione dei temporali nell'emisfero nord e avremmo desiderato essere on line un paio di mesi prima con questo argomento ma, dal momento che le condizioni meteorologiche sono la manifestazione della complessità del sistema atmosfera, il problema è di continua attualità. Quelle che seguono sono esperienze di piloti di linea negli Stati Uniti e sono riferite attraverso il programma Aviation Safety Reporting System.

Lo scopo per cui vengono pubblicate è quello di aumentare la consapevolezza di quanto possa essere difficile determinare l'entità di certi eventi atmosferici e valutare la possibilità di operarvi.

Ancora una volta la risposta è una filosofia conservativa, una consolidata strategia del margine.

The following pilot reports were selected from a NASA ASRS research:

Operational incident involving thunderstorm encounters in the airport terminai area.

Each pilot narrative is reprinted here to highlight the potential dangers of operating in the terminai area during thunderstorm activity.

WIND SQUALL ENCOUNTER DURING NIGHT TAKE-OFF

Thunderstorms were ten miles north-west of the airport and moving toward the field. We were waiting for take-off sequence on runway 14L at O'Hare lnternational airport.

The wind was reported as 180 degrees at 16 to 24 knots. When we were number two for take-off, a report of 180 degrees at 16 knots variabie 230 degrees with gusts to 24 knots was issued with a windshear alert in the south-west quadrant of the field.

We elected to use maximum EPR and attain additional airspeed prior to rotation. We were cleared for take-off. At around 110 knots, the tower reported gusts up to 57 knots from the south-west or west and a windshear alert in all quadrants. The co-pilot set maximum throtties and up to full rudder was required to stay on the runway. The airspeed hung after VR. We rotated near the end of the runway at "V2 plus", and continued to accelerate after lift off. The take-off roll was very rough due to wind buffeting and rudder inputs. I doubt if we could have stayed on the runway if we had elected to abort. lf it were daylight, we probably would have seen the visual clues of the wind squall coming across the airport and would not have started the take-off.

FIRM LANDING FOLLOWING UNEXPECTED 30 KNOT LOSS AT 100 FEET

We were landing at Miami lnternational airport and expecting an approach clearance for runway 9L or 9R. Passing 16,000 feet, the airport operation was turned for landings on runway 27R or 27L. We were vectored to the final approach for runway 27R, and followed another jet aircraft as we started the approach at 3000 feet. Light rain and thunderstorms were south of field. Visibility was about ½ to 1 mile. The aircraft ahead of us reported a loss of 10 knots in airspeed passing 800 feet in moderate rain. They landed and reported no problems. At 1800 feet, we encountered moderate rain and at 800 feet we did not encounter any loss of airspeed. At 500 feet winds were reported by tower to be 280 degrees at 13 knots with visibilities of one mile east of field, and more than 6 miles west of field. Passing through 150 feet, rain was heavy and the tower reported winds of 170 degrees at 13 knots. Our approach speed called for 142 knots. The first officer was fiying the aircraft at 162 knots. Right after the 100 feet call out was made, airspeed went from 162 knots to 125 to 130 knots. Full power was applied but firm contact with the runway was made.

MACHO MENTALITY RESULTS IN PENETRATION OF SEVERE THUNDERSTORM

The flight proceeded without incident until west of SCURRY VOR where we encountered a line of severe thunderstorms extending from SCURRY to well west of the Dallas Ft. Worth lnternational airport terminal area. I was the first officer and pilot flying. Over my objections and the flight engineer's, the decision was made to penetrate the thunderstorms. At an assigned altitude of 5000 feet on the approach segment, we experienced contouring cells at 12 o'clock followed by severe turbulence, altitude deviations of ±800 feet, airspeed deviations of ±30 knots, lightning, hail and windshears. All confirmed by ATC in advance. ATC further advised as we started final approach segment that an aircraft ahead of us had executed a missed approach and there was a heavy thunderstorm over the field. Again over our objections the decision was made to continue. At 500 feet, I executed a missed approach because of marginal aircraft control due to windshear and severe turbulence. At 3000 feet, assigned on missed approach, unable to hold altitude, we advised ATC. We finally diverted to Abilene to refuel. I would guess that 99.9 percent of our captains respect severe weather and avoid it at all costs. Unfortunately, there are a handful who still harbour macho attitudes that put the aeroplane, crew and passengers in peril.

IF YOU DON'T CRASH, IT MUST HAVE BEEN SAFE!

Approach control advised us from 40 miles out of Pittsburgh to base leg that there would be a thunderstorm very near, if not right over the field, at our estimated time of arrival.

They advised there were numerous lightning strikes. The winds on base leg were 140 degrees at 6 knots, and on short final they were 230 degrees at 18 knots. Everybody else was holding to let the weather pass. Our captain was going to land no matter what. As we crossed the numbers at 180 knots (about 45 above bug speed) we flew into a wall of water. Luckily we landed without incident. During our approach, ATC asked several (at least three) times what our intentions were. As we pulled into the gate, all ramp personnel had been cleared due to lightning. I told the captain I did not approve of his decision and felt he operated the aeroplane in an unsafe position and I got off the trip. During our arrival phase, I suggested we land at Cleveland or hold somewhere and let the weather pass. He more or less ignored me and landed with 14.1 tonnes of fuel, that is 2 hours. I do not know why he felt so compelled to land.

Also besides landing with a thunderstorm over the field, he penetrated the line to get to the airport. The line was level 2 and 4s, showed heavy red on the radar on the 50 mile range. The captain told me to hang on and expect heavy turbulence and hail. I still cannot believe his judgement. The autopilot held our altitude within 300 feet of the assigned 5000 feet. Once we broke out of the line, he flew 300 knots at 5000 feet to try to beat it to the airport. This failed, we arrived at the same time. The captain seemed to think since we made it without crashing, it must have been safe and his judgement sound.

LANDING REQUIRED FULL CONTROL INPUT, BUT AIRCRAFT STILL DRIFTS OFF HARD SURFACE

After being vectored around cells and placed on approximate 10 mile final for runway 36 R, we were cleared for a visual approach. We noticed a rain shower to the left of the runway as we approached. The first officer commented to the captain that a go around may be required if the weather moved toward the runway. The wind was reported from the west. The first officer requested and received missed approach instructions from the tower. The approach was continued with our feeling that the shower would not be over the field prior to our arrival. Normal procedures and call outs were used to 20 feet when moderate turbulence was encountered. At, or just prior to touchdown, the forward visibility became nil to none due to heavy rain which engulfed the aircraft. The wind became much stronger from the left and I noticed the captain with full control input for the wind and the aircraft still drifted toward the far right side of the runway. Upon post-flight inspection of the aircraft, grass and mud were seen on the right wheel well. Any further occurrences of this type of weather situation will be handled with a precautionary go around and hold until the weather improves.

REPORTER SAYS HIS CAPTAIN NEEDED A MORE CONSERVATIVE ATTITUDE AND GO AROUND

There were heavy thunderstorms over the field and in the airport's vicinity. I was pilot fiying and asked the captain if we could slow down to see what the field conditions were going to do and he said to keep the speed up and continue to the airport. Approach turned us onto the localizer to fly inbound at approximately 22 miles out. We were told to slow to 170 knots, which I did as the pilot fiying. The tower issued several windshear alerts and stated that the tower visibility had dropped to ½ mile with very heavy rain showers. Tracking inbound at 170 knots, the airspeed indicator suddenly shot up to 225 knots and the aircraft rose 300 feet. The airspeed then dropped off at a gradual rate. Turbulence was severe, but I was able to maintain glideslope and localizer as we were cleared for an ILS approach. At minimums, the aircraft was blown to the right side of the localizer past window tolerances. I was using an eight degree bank to manoeuvre back toward the runway, but the track was not correcting. The captain said, "I have control" and I relinquished the aircraft to him. As I relinquished control, I glanced down at the radar and there was no going around, as the screen showed pretty much solid red in the departure path. Tower gave us several windshear alerts as we approached the runway. The captain wrestled with the aircraft and got it over the centreline and on the ground with what I estimated to be 1000 feet of white runway lights remaining. He immediateiy applied full braking pressure and brought the aircraft to a stop at the end of the runway. Later that night, the captain asked what my thoughts were on the event and I replied, "I would have taken a more conservative approach to weather".

INNOCENT LOOKING CLOUD BANK DOUSES FLIGHT WITH HEAVY RAIN - FULL POWER REQUIRED TO ESCAPE

Approaching Atlanta's Hartfield airport, weather was reported 4000 feet ceiling in three miles visibility. After passing the LOM and at approximately 800 to 1100 feet, we entered a small cloud bank. I went back on my instruments, then I saw the approach lights and approach environment so I was both in and out of the cockpit when it seemed like somebody threw a huge bucket of water over us. We had experienced a sudden rain shower that was heavy. I transferred back to my instruments and discovered that we were below glidepath 1 dot to 1½ dots low. I corrected immediately, then we went below again. This time I realised it was not a good situation so I executed a go-around. lt took full power to come out of this situation. Then, all of a sudden, we were clear - just like nothing had happened. Quite an experience that makes one grow old faster.

HEAVY WORKLOAD DURING WEATHER AVOIDANCE CAUSES CREW TO MISS ATC CLEARANCE

We were cleared for take-off and immediately entered rain and turbulence. Weather radar and coast approach were very busy. We remained on runway heading to 4000 feet when ATC asked us if we had started our turn to our assigned heading. I believe we missed making our assigned turn at 1500 feet due to a heavy workload in the cockpit at the time, along with focusing our attention on the radar for weather avoidance.

CREW ASTONISHED AS THEY ENCOUNTER EXTRAORDINARY DOWNDRAFT

Our crew was attentive to a fast moving thunderstorm approaching from the north-west. This was a single but large cell contouring on radar when the crew arrived at the airport. The thunderstorm was approximateiy 15 to 20 miles from the airport. Our turn time on the ground was only 15 minutes or so as we pushed back for departure. The cell was now 5 to 7 miles from airport and winds had shifted to west-northwest. Runway 27 was active. The crew requested and received clearance for immediate left turn to a 180 degree heading after take-off. Winds were light and steady and take-off was into clear air with clouds outdropping from a cell at approximately 2000 feet AGL. The sky was clear to the south. A normal take-off was made, but adverse condition profile was used as a precaution with flaps raised earlier than normal. Profile and initial climb speed of 210 knots was established. The captain was flying and was attempting to make a left turn as assigned. Significant forces were applied to the yoke but no roll response, even to the point that the first off icer stated "you better start the left turn". The captain replied.. "l'm trying". Several seconds passed before roll response allowed the left turn. Passing through approximately 220 to 230 degree heading, moderate turbulence was encountered and the aircraft was accelerated to 230 knots. Roll rate and turn rate decreased to allow an airspeed increase. Some gusts were encountered. At this point we were probably 1000 to 1200 feet AGL. Somewhere around a 200 degree heading, with constant pitch angle, airspeed increased to over 300 knots instantaneously and vertical speed became indeterminate as VSIs were completely pegged at a 6000 fpm rate of climb. At this point the ride was fairly smooth.

The astonished crew tried to analyse the events and decided that immediately after take-off the right wing must have been caught in a severe downdraft preventing the turn. Then, turbulence was encountered between the downdraft and outflow and then airspeed increase of 70 to 180 knots occurred entering the horizontal vortex. Updraft speed as stated, was immeasurable and unbelievable. Obviously we were very fortunate. No doubt exists in my mind that had we encountered the same circumstances in reverse with a 70 to 100 knots airspeed loss and extraordinary downdraft forces, it would have been impossible for this aircraft or any other to have flown through the encounter and to have survived.

EMERGENCY DECLARED AS CREW HAD NO PLACE TO GO

We were flight planned to Miami lnternational airport. Throughout the flight we monitored VOLMET on HF frequency for weather of the Miami area as well as SIGMET reports. Throughout the flight, the VOLMET reports were fairly consistent with the forecast of rain showers. When we reached the southern shore of Cuba at the navaid fix UCL, the convective activity had increased twofold. We were unable to fly through a line of thunderstorms and deviated about 30 miles to the east of our routing. Headwinds began to pick up rapidiy, reducing our ground speed. We then showed an overburn of 1.0 tonne and had 1.1 tonnes above flight plan remaining fuel. The squall line had some slight openings to be able to fly through and it appeared that the line was circling the Miami airport area. When we initiated our descent, thunderstorms began to mature at an unbelievable rate forming a squall line east of our position towards Nassau. We requested weather for Ft.Lauderdale and West Palm Beach. Ft. Lauderdale had a power outage and West Palm Beach had severe weather in progress. We calculated that we would not be able to make Nassau due to the intensity of the squall line and the low amount fuel we would have upon arrival into Nassau. At approximately FL 200, we were advised to hold south-east of JUNOR as published at 11,000 feet. Due to our fuel status, we requested an EFC (Estimated Further Clearance) time. We had briefed for an ILS 27R, as well as plans if a missed approached were initiated. Upon entering the hold, we had 10.6 tonnes on the fuel. The first officer was advised to state minimum fuel to ATC. Before we had completed the outbound leg of the hold, we were cleared direct Miami. Due to the severe weather we opted to turn right instead of left to fly direct. We began encountering moderate turbulence and the weather was intensifying around JUNOR intersection. We were penetrating the lighter weather on radar but realised there was no way we could turn back easterly. At this point, we had approximately 8.6 tonnes on our fuel remaining. We were advised Miami airport had closed again due to weather, and we were cleared to turn back to JUNOR intersection and await further instructions. I had no choice but to tell my first officer to declare an emergency. We would not be able to fly through the severe squall line and had no alternate airport available and our fuel level was too critical to even consider any alternative. We were again cleared to Miami airport and landed without incident. The weather was not bad at the airport. We broke out at 1500 feet with visibility four miles with rain and lots of wind. We landed with 6.8 tonnes and blocked in with 6.6. We had landed shortly after the severe winds and rain went through.

FOLLOWING THIS INCIDENT THE PILOT GAINED A HEALTHIER RESPECT FOR FAST MOVING THUNDERSTORMS

Enroute, the company advised of a line of weather moving into JFK area and we should be ahead of the line for arrival. We were vectored for the VOR/ DME 13L approach then re-sequenced for the ILS 22 L approach due to thunderstorms along VOR/ DME 13L approach and moving toward the airport. On ILS, after being cleared for approach, we were advised of windshear on the field. Glideslope flags appeared and we initiated a missed approach with immediate turn to avoid thunderstorm cell. During vectoring, we went through some moderate turbulence on climb out. We declared minimum fuel and landed at Philadelphia airport ahead of a second line. What would I change? My respect for fast developing, fast moving thunderstorm cells is much healthier. I would plan for more margin when conditions are right for their development.

WEATHER DISTRACTS CREW AND AIRCRAFT ALMOST LANDS GEAR-UP

Heavy rain showers moved over the field obscuring visibility. Radar painted the area red (heavy precipitation), but because a wide body aircraft landing ahead of us reported no problems, we continued the approach. Slowing to 170 knots per approach control, I commanded flaps 3. Usually, I select gear down at flaps 3, but we were so far out, I elected to keep it retracted to save fuel and vibration on the airframe. As we approached the outer marker, I commanded full flaps and final items to be read on the checklist. At the marker, we were in heavy rain with light to moderate turbulence. I became very concerned about what we were about to fly into. I kept asking the first officer for weather reports. Air traffic control seemed indifferent and painted a rosy picture. We continued the approach. At 700 feet AGL and heavy, heavy rain, we received a cockpit warning - our gear was up! lmmediately, I commanded gear down and final items. At 500 feet the gear was down and locked, and we broke out of the rain and got a visual on runway 29 L, landing uneventfully. The first officer was so concerned about the weather that he forgot to read the checklist. To the pilots who think this could never happen - it can!

A SMALL RAINSHOWER ENCOUNTER RESULTS IN 50 KNOT LOSS OF AIRSPEED

It became apparent that a small line of weather was approaching the field. We watched on radar as level 2 and 3 storms approached. Approach control advised field conditions were still VFR in light rain and some thunder. Another aircraft on final ahead of us reported a smooth ride all the way down final. The vector to approach was poor. We crossed the marker one dot off localizer. At 1000 feet AGL we were still trying to get back on the localizer when the airspeed increased and the glideslope went to full down. The first officer was pilot flying and initiated a go-around. We climbed straight ahead to 2300 feet on our way to 3000 feet. Airspeed had increased to approximately 240 knots due to shear. Then airspeed decreased to 190 knots. Stall warning and stick shaker activated. We put power up to go around thrust or more. Pitch attitude was approximately 0 to 5 degrees down. Altitude was decreasing and airspeed was stable at 200 knots. Any increase in pitch brought on stick shaker. We stopped the descent at 1600 feet. We waited to the west of the airport until weather had passed, and then made a visual approach and normal landing. Had we been on localizer and glideslope and continue approach with landing configuration, I do not believe we would have been able to recover. For a relatively small rainshower, we received a 50 knot loss of airspeed and a 700 foot loss of altitude. A real eye opener!

CREW FEEL THEY ENCOUNTERED A MICROBURST WHILE EXECUTING A GOAROUND

Thunderstorms appeared to be heavy slightly southeast to approximately 20 mile north-east of Washington D.C. The first approach controller stated he did not have a good return of this weather on his scope and could not see what we did. The final controller did see this band of weather and vectored us west of the storms. The ATIS was reporting scattered to broken with .07 miles visibility. Wind was 200 at 0.7 knots. I briefed for the approach at approximately 13 to 14 miles northwest of the airport as we were descending to 2000 feet on vectors. Someone then transmitted in the blind and stated, "moderate to heavy rain at DCA". We intercepted the localizer at 2000 feet, eight miles out. We could see the airport and runway lights. The tower said, "wind 230 / 4 to 5". My co-pilot and I both said, "did he say 4 to 5 or 45 knots?" The last was correct. My co-pilot then asked, "wind?" The tower responded with 230 at 30 knots. We could still see the airport lights, runway lights, and the VASI. I followed the approach procedure over the river at 3.5 miles with runway and VASI still in view. At approximately 1 ½ to 1 miles, and 400 to 500 feet, I lined up on runway 18. Immediately the aircraft was inundated with water. I immediately commanded a missed approach, added take-off power, flaps 15 degrees and gear up. Speed was approximately Vref +20 knots at the time. On pull up and go-around, we encountered heavy rain, large hail and moderate turbulence. Lightning was also present. For a short time at about 600 feet, with 15 degrees nose (body angle), the aircraft seemed to hesitate to climb, and speed deteriorated from about 170 knots to 138 to 140 knots. I immediately applied full power and the aircraft responded immediately. The second officer called, “number three is in red". Then he and the co-pilot both noted all three engines were in the red (both N1 and EGT). Exact readings were impossible due to the turbulence. I eased all three engines (throttles) back at about 1500 feet. After we initiated the missed approach, I told the tower we needed at least a 240 degree heading. We got instructions of 270 degrees and to maintain 3000 or 5000 feet if we wanted, and to contact Dulles Approach for holding instructions. We held and selected Dulles and landed uneventfully. The general opinion of the crew was that we might have encountered a microburst.

In conclusione un report sullo stesso argomento che abbiamo ripreso da un safety magazine della United Airlines. Contiene alcune interessanti riflessioni.

They Were Landing During The Microburst Alert!

I think SAFETYLINER readers will be interested in the following scenario which unfolded before my eyes as we were waiting for departure at O'Hare one summer evening a while back.

By First Officer M.M. Schaefer

It was a dark and stormy evening at O'Hare, not unlike many others we have seen come and go. The line of thunderstorms was beginning to make itself known around 19:30 local time just west of the field. The departures were leaving on 32 L, T/O and arrivals using 27R circle to 27L (ILS OTS). As we approached T/O for takeoff, it was becoming apparent that our route for escaping the O'Hare area and continuing to Richmond was not going to coincide with the available routes that Departure had for us to utilize.

The radar painted almost exclusively red every direction we maneuvered. Therefore, we declined takeoff clearance and were instructed to taxi across 32L into the 9R pad to join numerous other aircraft already there. As soon as we taxied across the runway, the rain began. It wasn't just raining, it was pouring. So hard, in fact, I put the wipers on high just to see in front of us. You could not see across the pad except for the light of the now numerous and consistent lightning strikes just off the west end and moving over the field. It was at this time that my workload subsided and I realized other aircraft were still landing!

Tower was calling wind shear alert - north boundary wind 350 at 10, east boundary 220 at 18. "Aircraft A ‑ inside Tafts." Tower: "Roger Airline A ‑, cleared to land 27R, wind shear alert ... (as I explained), "Roger, Airline A‑, cleared to land."

By now they were no longer shooting visuals to the left side ‑ I doubt tower could see the ground, let alone the runway. I looked at the Captain and said something to the effect "I can't believe guys are still landing in this stuff." We studied our radar again in disbelief.

Next came "United ‑ inside Tafts." "Roger United ‑, cleared to land, wind shear alert. . ." Without hesitation, "United ‑ going around." "Roger United ‑ fly heading ‑ (missed approach directions). I thought, thank God, now these guys will wake up and reevaluate the situation.

"Airline A ‑ inside Tafts" He gets the same brief and lands. Then tower called, "microburst alert, minus 35 knots on 3 mile final . . ." plus the wind shear reports as before on the field parameters. "Airline B ‑ inside Tafts" "Roger Airline B ‑ cleared to land 27R, microburst alert. . ." They land. Airline C, same brief, they land. United ‑ immediate go around. Airline D, lands. Another United goes around. Another Airline D goes around.

I also noted numerous aircraft stating "good rides all the way down" as they went to ground control seemingly lulling the next aircraft down the possible path of destruction. This was a MICROBURST ALERT with all the classic clues. Next, another United. Same brief, but somewhere on short final you hear, "Did you say microburst alert?" Tower was preoccupied with something else and never answered. They land! "An Airline B ‑ Tafts." Same warning. They land. I believe it was at this time (about 1950 local) tower quit calling "microburst alert" and warned only of wind shear activity. The rain subsided as the storm moved to the east side of the field. It was now about 1955 local. The tower announced winds calm.

I mentioned to the Captain when this all started that it sounded just like the wind shear training of our PC LOFT, the "A Windshear Avoided" tape kept replaying through my mind as I wondered what the other airlines had for wind shear training. I prayed no one would stub their toe on the runway, let alone roll a perfectly good aircraft into a ball in the weather like another airline did in Dallas, learning too late to avoid. This whole event sounded too much like a scenario on CNN the day after and I feared the luck of those landing that day could run out. On a happier note, it looks like United's training is working, for the most part.

One final note ...

After we were finally en route to Richmond, the captain, engineer and I were discussing what we had all learned from these events. The Captain expressed his concern for the lone United flight that landed during the microburst alert. He said it would have been interesting to hear him explain his actions and thoughts on the matter to his peers.