Appendix 1
2) Hourly Sequence Reports provide a strong indication that windshear may be present.
Hourly sequence reports should be inspected thunderstorms, The temperature and for windshear rainshowers, clues-- or MSO 007/83/45/1715G30/974 VSBY W3 ST/2 SA 2152 50BKN 800VC 12TBD Dlowing dust.
ed for large differences, indicat- dew point spread should be examin- 1.e. 30 3) Severe Weather Watch Reports to 50 degrees Fahrenheit, A check for issuance of an aviaing Tow humi dity. Additional tion severe weather watch should signs such as warming trends, be made. In the following examgusty winds, cumulonimbus clouds, ple, a line of thunderstorms apetc., should be noted. proaching severe limits are forecast. Severe convective weather In the example shown below, evi- is a prime source for microbursts.
dence of blowing dust, large temperature/dewpoint spread, gusting ISTMS FCST TO APCH SYR LIMITS THIS winds, and thunderstorms (without AFTN AND ERY EVE TO THE RT OF A LN rain) is present. These signs FM YUM IPL RIV BUR 15 W SDB BFL IYK EED YUM.
Appendix 1
4) LLWAS Reports LLWAS in its present form has some limitations:
LLWAS (Low Level Windshear Alert System) - Presently installed at A) shear magnitude may be serious- 110 airports in the U.S., this ly underestimated, system is designed to detect wind B) trees, buildings and other surshifts between outlying stations face obstructions may disrupt and a reference centerfield sta- (Figure 26) by or limit the flow of air near tion If an LLWAS the sensors, alert (triggered wind speed C) location and spacing of sensors occurs, it indicates the presence and/or direction differential) may allow microburst develop- ment to go entirely in undesomething shear-like, though tected--particularly the not necessarily indicative of mag- early stages (Figure 26), nitude or location. However, the D) because the sensors are on the absence an alert does not ground, microburst development necessarily indicate that it is which has not yet reached the safe to proceed! surface will not be detected, and LLWAS information is available by E) LLWAS coverage only exists near request anytime, but will be pro- the runways and typically does vided by controllers whenever an not extend beyond the middle LLWAS alert is in progress. marker.
WINDSHEAR ALERTS. 2 QUADRANTS. Even with these limitations, LLWAS CENTERFIELD WIND 210 AT 14. WEST can provide useful information BOUNDARY WIND 140 AT 22. NORTH- about winds around the airport.
EAST BOUNDARY WIND 270 AT 24.
LLWAS Stations
• Runways Microburst 3 nmi 2 nmi 1 nmi Figure 26. Undetected microburst due to wide LLWAS station spacing.
Appendix 1
5) SIGMETS ring of blowing dust or localized dust in general, flying debris, SIGMETS, particularly CONVECTIVE virga, a rain core with rain di- SEvE STrang worms of scattared provide essential verging away horizontally from the clues. (fun i clouds, to tornadose tures (funne) tornados).
embedded thunderstorms, light, lightning may be the onl reaching level 5 intensity, indi- isual clue. Pilots must become cating a potential for windshear. aware that these visual clues are often the only means to identity ATTENTION ALL AIRCRAFT. CONVECTIVE the presence of severe windshear.
SIGMET CHARLIE ONE FROM THE VICIN- TTY OF ELMIRA TO PHILLIPSBURG. 7) PIREPS
SCATTERED EMBEDDED THUNDERSTORMS MOVING EAST AT ONE ZERO KNOTS. PIREPS are extremely important in- FEW INTENSE LEVEL FIVE CELLS, MAX- dicators in microburst windshear IMUM TOPS FLIGHT LEVEL FOUR FIVE Reports of sudden ZERO. irspeed changes in the airport approach or landing corridors pro-
6) Visual Clues from the Cockpit vide indication of the presence of windshear.
The value of recognizing microbursts by visual clues from the Mountered wingshear fin final, cockpit cannot be overemphasized. encountered final, Pilots must remember that micro- loss of 20 knots at 300 feet apbursts occur only in the presence proaching middle marker runway 27 of convective weather indicated by right, Douglas MD-80."
cumulus-type clouds, and virge. thunderstorms that other types of wind- rain showers, In international weather reports, (Note windshear observations or reports shear can occur in the absence of from pilots may be included at the convective weather.) end of routine and special weather reports.
Microburst windshear can often be identified by some obvious visual ON ANCH RY 347A7 16EORTED MOD MS clues such as heavy rain (in a dry or moist environment). This is particularly true if the rain is In the above example, a Boeing 727 accompanied by curling outflow, a on approach to Runway 34 reported moderate windshear at 1610 hours.
Appendix 1
8) Airborne Weather Radar Another significant aspect of weather radar use is attenuation.
The use of airborne weather radar Attenuation is caused by heavy to detect convective cells should rainfall reducing the ability of De considered a matter of rou- the radar signal to penetrate, tine. Weather radar provides causing the radar to present an extremely useful information for incomplete picture of the weather the avoidance of thunderstorms in area. In the terminal area, comthe airport terminal area, but parison of ground returns to cannot directly detect windshear. weather echoes is a useful tech- Pilots have become adept at avoid- nique to identify when attenuation ing thunderstorms while enroute is occurring. Tilt the antenna and at altitude. However, rela- down and observe ground returns tively little emphasis has been around the radar echo. With very placed on their use near the heavy intervening rain, ground reterminal area. Most heavy rain turns behind the echo will not be De thunderstorms near the airport can detected with conventional peturns is referred to as a shadow This area lacking ground weather radar by a careful use of and may indicate a larger area of tilt control to scan above the precipitation than is shown on the intended flight altitude (15,000 indicator. Areas of shadowing to 20,000 feet). The subject of should be avoided.
the proper use of tilt control for this purpose should be studied in Turbulence Doppler radars starting available radar manuals and to become available to flight bulletins. crews provide an additional clue to the presence of microbursts.
Pilots should understand that If, in the departure/arrival segneavy precipitation, as seen on ment of the airport vicinity, turtheir radar and associated with bulence indications are present in onvective clouds indicates the mid-levels of convective cloud ossibility of microbursts; now- systems (e.g., 15,000 to 20,000 ever, the absence of strong re- feet) and low humidity conditions indicate a safe situation. turns must not be depended upon to Poten- spread), the situation is pilots exist (large temperature/dewpoint prime tially hazardous dry microburst for microburst formation.
conditions may only produce weak should be particularly cautious radar returns. when they observe this indication.
11/25/88
2.4.2 AVOID KNOWN WINDSHEAR The importance of avoiding severe windshear and microbursts cannot be over-emphasized. Microburst windshears have been measured which are beyond the capability of transport category airplanes and the most highly skilled pilots. Recall from Figure 11 that even windshears which were within the performance capability of the airplane have caused accidents. Avoidance may only involve delaying departure or approach for 10 to 20 minutes since this is the typical time required for microburst dissipation.
Even though significant emphasis on simulator training is recommended in ilot training curriculums, avoidance nust be the first line of defense.
Simulators are valuable for teaching windshear recognition and recovery.
However, pilots are cautioned not to develop the impression that real-world windshear encounters can be successfully negotiated simply because they AC 00-54 Appendix 1 have received simulator training. In an airplane, complicating factors
(l.e. turbulence, precipitation noise, instrument errors, etc.) may make shears much more difficult than in a simulator. In addition, simulator motion systems are limited in their capability to reproduce all the dynamics of an actual windshear encounter.
Remember, some windshears cannot be escaped using any known techniques.
Therefore, above all, AVOID, AVOID, AVOID:
Unfortunately, there are no universal quantitative windshear avoidance criterno-go thecision guidelineambiquere assured detection and warning system in operation which can measure windshear intensity along a specific the weather evaluation factors which factors which can be helpful in avoiding windshear is shown in Table 1 on the following Page 35
11/25/88 AC 00-54 Appendix 1 TABLE 1
MICROBURST WINDSHEAR PROBABILITY GUIDELINES
OBSERVATION PROBABILITY
OF WINDSHEAR PRESENCE OF CONVECTIVE WEATHER NEAR INTENDED FLIGHT PATH:
With localized strong winds (Tower reports or observed blowing dust, rings of dust,
tornado-like features, etc.) ....HIGH
With heavy precipitation (Observed or radar indications of contour, red or attenuation shadow).. HIGH With rainshower MEDIUM With lightning MEDIUM with virga MEDIUM With moderate or greater turbulence (reported or radar indications) MEDIUM with temperature/dew point spread between 30 and 50 degrees fahrenheit. MEDIUM ONBOARD WINDSHEAR DETECTION SYSTEM ALERT (Reported or observed). HIGH PIREP OF AIRSPEED LOSS OR GAIN:
15 knots or greater HIGH Less than 15 knots MEDIUM
LLWAS ALERT/WIND VELOCITY CHANGE 20 knots or greater HIGH Less than 20 knots MEDIUM
FORECAST OF CONVECTIVE WEATHER LON NOTE: These guidelines apply to operations in the airport vicinity (within patien de ere he d is oved the prostity ed ring should be increased. The hazard increases with proximity to the convective weather. Weather assessment should be made continuously.
CAUTION: CURRENTLY NO QUANTITATIVE MEANS EXISTS FOR DETERMINING THE PRESENCE OR INTENSITY OF MICROBURST WINDSHEAR. PILOTS ARE URGED TO EXERCISE CAUTION IN DETERMINING A COURSE OF ACTION.
11/25/88 AC 00-54 Appendix 1 Table 1, designed specifically for in off pates surel convective weather (thunderstorm, terms path, such enraves evaluation of roarious suerva- counters may present other significant evaluation various observa- weather related risks. Pilots are tional clues to aid in making appro- therefore urged to exercise cautior priate real time avoidance decisions. The observation weighting is categor- when determining as course of actione Use of Table should not replace ized according to the following scale: sound judgement in making avoidance decisions.
HIGH PROBABILITY:
Windshear clues should be considered Critical attention need be given to cumulative. The probability for each this observation. A decision to avoid single observation is given. However,
(e.g. divert or delay) is appropriate. if more than one windshear clue is observed, the total probability rating MEDIUM PROBABILITY: may be increased to reflect the total set of observations.
Consideration should be given to avoiding. Precautions are appropriate. Example:
LOW PROBABILITY: Nearing destination based clouds over VIRGA is seen descending from high Consideration should be given to this the airfield (MEDIUM PROBABILITY).
observation, but a decision to avoid Commencing approach, a PIREP is reis not generally indicated. ceived indicating that another flight just experienced a 10 knot airspeed The guidelines in Table 1 apply to airport (MEDIUM approach loss on final to the same operations in the airport vicinity PROBABILITY). There- (within 3 miles of takeoff or landing tore, it would be appropriate to raise along the intended flight path below the total avoidance decision weighting 1000 feet AGL). Although encountering to HIGH PROBABILITY (indicating a weather conditions described in Table decision to avoid is appropriate).
Appendix 1
2.4.3 CONSIDER PRECAUTIONS altitude available for recovery if required. Lastly, full thrust takeoffs Avoidance is the best precaution. may eliminate resetting thrust in a However, there are situations when recovery, thereby maximizing accelerawindshear clues do not clearly dictate tion capability and reducing crew delaying, but can be interpreted to workload.
mean that conditions are right for windshear activity. In these in- • Runway Selection stances, pilots should consider the next step of flight crew actions, the Use the longest suitable runway that use of precautions. avoids suspected areas of windshear.
The choice of a suitable runway in- A number of precautionary techniques volves consideration of exposure to have been developed which crews can obstacles after liftoff and crosswind take to lessen the effect of windshear and tailwind limitations.
should an unsuspected severe windshear sures maximum runway available to acbe encountered on takeoff or ap- celerate to rotation speed and may reproach. These precautions include sult in more ground clearance at the consideration of thrust setting, run- end of the runway and during the climb way selection, flap selection, air- profile. Should the decision be made speed, use of autopilot, autothrottle to reject the takeoff, more runway is and flight director. They were devel- available on which to stop the airoped by detailed analysis and piloted plane.
simulation of several microburst windshear encounters. In many cases, • Takeoff Flap Selection tradeoffs were involved and no "best" recommendation for all conditions The choice of takeoff flap setting is could be developed. dependent on the airplane type. The following flap settings should be con- Use of precautions along with even the sidered unless limited by obstacle best recovery piloting skills cannot clearance and/or climb gradient:
guarantee a successful escape from any microburst windshears. It is im- Airplane Takeoff ortant to realize that the recom- Type Flap Setting mended precautions each have a relatively small effect on the outcome of B727 15 an inadvertent windshear encounter. B737 5 to 15 Therefore, use of precautions should B747 20 not replace sound pilot judgment in B757 20 deciding whether or not it is safe to B767 20 proceed. Use of precautions should not bias a go/no-go decision in the go 10 or 20 direction. DC-9-20, -30,-40,-50 5 or 15 MD-80 5 to 15
2.4.3.1 TAKEOFF PRECAUTIONS DC-10 5 to 20
• Thrust Setting L-1011 10 to 22 Maximum rated takeoff thrust should be Studies of available takeoff flap setused for takeoff. This shortens the tings showed that the greater flap takeoff roll and reduces overrun ex- setting provided best performance for posure. Full thrust also provides the windshear encounters on the runway.
best rate of climb, thus increasing However, lesser flap settings showed Page 38
Appendix 1 the best performance for in-air wind- 1) Determine 'actual and Vz shear encounters. The takeoff flap speeds for airplane settings in the table offered somewhat gross weight and flap setting.
better performance over a wide range Set airspeed bugs to these of conditions; however, it must be values in the normal manner.
pointed out that the performance difference between flap settings is small. 2) Determine field length limit maximum weight and correspondo Increased airspeed ing Vp for selected runway.
Increased airspeed rotation 3) If field length limit VR is proves the ability of the airplane to greater windshear weight VR, after liftoff. Increased airspeed im- Vr lup to 20 knots in excess proves the flight path, reduces poten- actual weight VR) flight takeot be Airseted speeds, and reduces pilot work- should to the higher speeds.
Delaying rotation to a higher airspeed 4) Rotate to normal initial climb may appear to increase the risk of attitude at the increased attitude. VR overrunning available runway. How- and maintain this ever, Decause of the manner in which This technique produces increased rotation speed is calcu- higher initial climb speed lated, it is simply using the runway which slowly bleeds off to the as if the aircraft was loaded to the normal initial climb speed.
field length limit weight for that runway limit conditions, If the takeoff is at field WARNING: If windshear is encountered length the risk of beyond the actual overrunning the available runway is gross weight (bug) YR, do increased because there is no extra not attempt to accelerate to runway available. The overrun expo- the increased VR, but rosure is also increased if the wind- tate without hesitation. In shear reduces the airspeed below the no case should rotation be minimum airspeed required for liftoff delayed beyond 2,000 feet at the maximum available (body con- from the end of the usable tact) attitude. However, initiating runway surface. ( See Secrotation no later than 2000 feet from tion 2.4.5 Recovery Techthe end of the usable runway surface niques) reduces the probability of overrun and liftoff. maximizes the available energy after If increased than normal accelerating liftoff, airspeed was not used airspeed If increased Vp is to be used, the Reducing technique for scheduling and using in- pitch attitude creased rotation airspeed is:
windshear is encountered.
Appendix 1
• Flight Director rectors must be disregarded if a recovery is required not use speed-referenced flight and, time permitting, directors unless they equipped switched off by the Pilot with windshear recovery guidance. Not Flying (PNF).
WARNING: speed-referenced flight Some flight directors are equipped director which does not have with a selectable pitch attitude windshear recovery guidance mode. feature, the If normal procedures utilize may command a pitch attitude this selectable pitch change to follow target attitude mode may be effectively used airspeeds regardless of in a windshear encounter provided the flight path degradation. selected attitude is within the ac- This guidance may be in ceptable range. However, if an atticonflict with the proper tude other then the selected attitude procedures for windshear becomes necessary, the flight director recovery. Such flight di- should be disregarded, and time permitting, switched off by the PNF.
Takeoff Precautions
• Use maximum rated takeoff thrust
• Use longest suitable runway
• Consider using recommended flap setting
• Consider using increased rotation airspeed
• Do not use speed referenced flight director Page 40
Appendix 1
2.4.3.2 APPROACH PRECAUTIONS considered:
• Stabilized Approach Airplane Landing During some normal operations achieved stabi- Type Flap Setting lized prior to 500 feet AGL. environment, approaches However, in a B727 B737 30 30 potential windshear a B747 25 or 30 stabilized approach should be estab- B757 30 lished no later than 1,000 feet AGL to B767 30 improve windshear recognition capability. DC-9 * MD-80 28
• Thrust Management DC-70 35 Minimize thrust reductions. L-1011 33 than immediately compensating for an airspeed increase by reducing * Minimum flap setting authorized for a brief pause to evaluate speed trends particular model.
occurs and recovery is initiated, thr is prudent.
Studies of windshear encounters using additional airspeed and earlier avail- a11 available landing flap settings ability of thrust (due to engines ac- showed that the flap settings recomcelerating from a higher RPM) will be mended above provided the best overall advantageous. recovery performance for a wide range assure inappropriate thrust of windshears.
reductions do not occur.
sence of a tailwind shear, • Increased Airspeed cedure may result in a higher Increased airspeed on approach imto be accounted for on landing. proves climb performance capability and reduces the potential for flight
• Runway Selection at stick shaker during recovery from an inadvertent windshear encounter.
avoids the area of suspected windshear If available landing field length perand is compatible with mits, airspeed may be increased up to A longer runway a maximum of 20 knots. This increased provides the greatest margin for in- fouchdown must emar wathin the flamai speed should be maintained to creased ground roll due pated winds and speed Possible to unantici- resulting touchdown zone--do not allow the airplane to float down the runway.
(instrument) other aids to glide path monitoring As many variables are involved, it is (VASI, etc.) are desirable as not practical to provide exact guidthey can enhance windshear recognition ance on the effect of 20 knots extra providing timely, speed actual stopping distance.
path deviation information. Wind can be a major factor since stopping distance is affected by ground-
• Landing Flap Selection speed rather than airspeed. If increased airspeed is used and an in- The choice of landing flap setting is creasing performance shear is encoundependent on airplane туре. The tered, a go-around may be necessary following flap settings should be due insufficient landing field Page 41
Appendix 1 length for the higher approach speed. 25 percent and in some cases Furthermore, if a pilot can be reason- may exceed brake energy ably certain that wind changes (due to limits.
topography or unique local conditions) will not result in decreasing perform- • Flight Director and/or Autopilot ance, it may be inappropriate and Autothrottles increased approach speed.
During approach it is desirable to Other factors affecting stopping dis- utilize the flight director, autopilot tance such as availability and effec- and autothrottles to the maximum extiveness of thrust reversers, tire and tent practical. These systems may rebrake condition, runway surface conditions, etc., must also be taken into crew more time to monitor instruments consideration. On a dry runway with and weather conditions.
field length may accorresete 20 knots of autoflight systems, and in particuthe autothrottle, only extra speed benefits if properly monitored.
cases greater field length may be absence of proper monitoring, in doubt, these systems may mask onset of shear longest suitable runway which does not awareness of expose the airplane to greater hazard control inputs being made.
from possible shear.
Furthermore, not all autoflight sys- WARNING: Increased touchdown tems perform well in gusty or turbuincrease stopping distance. lent conditions. The autopilot and/or additional knots at the autothrottle should be discontouchdown can increase stop- nected when continued use appears ping distance by as much as counter-productive.
Approach Precautions
• Stabilize approach no later than 1000 ft AGL
• Minimize thrust reductions
• Use most suitable runway
• Consider using recommended flap setting
• Consider using increased approach speed
• Use autoflight systems during approach Page 42