REL­E­VANT DIS­CUS­SION: AIM 4-3-26, 7-1-1, 7-1-11, 7-1-17, 7-1-29, 7-1-30, FAA-H-8083-16, 8083-25, AC 150/5220-16, OR­DER 8260.19F 8-6-6

Flying - - CONTENTS - By Richie Len­gel

AWOS: Au­to­mated weather ob­serv­ing sys­tem. A suite of weather sen­sors of many dif­fer­ent con­fig­u­ra­tions ei­ther pro­cured by the FAA or pur­chased by in­di­vid­u­als, groups or air­ports that are re­quired to meet FAA stan­dards. AWSS — au­to­matic weather sen­sor sys­tem — is func­tion­ally the same as ASOS.

1. A state or air­port can pur­chase an AWOS with as­sis­tance from the FAA’s Air­port Im­prove­ment Pro­gram. 2. The sys­tem trans­mits a 20- to 30-sec­ond weather mes­sage up­dated each minute. 3. AWOS lev­els:

³ AWOS-A — Only re­ports the al­time­ter set­ting.

³ AWOS-AV — Al­time­ter and vis­i­bil­ity. ³ AWOS-1 — Al­time­ter, wind data (ref­er­enced to mag­netic north), tem­per­a­ture, dew point and den­sity al­ti­tude. ³ AWOS-2 — In­for­ma­tion pro­vided by AWOS-1 plus vis­i­bil­ity.

³ AWOS-3 — In­for­ma­tion pro­vided by AWOS-2 plus cloud/ceil­ing data. ³ AWOS-3P — Same as AWOS-3 and adds a pre­cip­i­ta­tion type iden­ti­fi­ca­tion sen­sor. ³ AWOS-3P/T — Same as AWOS-3P and adds thun­der­storm/light­ning re­port­ing. ³ AWOS-3T — Same as AWOS-3 and adds thun­der­storm/light­ning re­port­ing. ³ AWOS-4 — Con­tains all the AWOS-3 sys­tem sen­sors, plus pre­cip­i­ta­tion oc­cur­rence, type and ac­cu­mu­la­tion, freez­ing rain, thun­der­storm/light­ning and run­way sur­face con­di­tion sen­sors.

4. re­marks AWOS about can also den­sity gen­er­ate al­ti­tude, au­to­mated vari­able winds and ceil­ings.

5. A01 in­di­cates a sta­tion with­out a pre­cip­i­ta­tion iden­ti­fi­ca­tion sen­sor (i.e., can­not de­ter­mine the dif­fer­ence be­tween liq­uid or frozen/freez­ing pre­cip­i­ta­tion). 6. A02 in­di­cates a sta­tion with a pre­cip­i­ta­tion iden­ti­fi­ca­tion sen­sor.

7. AWOS in­for­ma­tion can be ob­tained on the proper fre­quency in flight and by tele­phone on the ground.

8. An AWOS com­mis­sioned be­fore July 1996 is not re­quired to use the metar for­mat. That means it might re­port tem­per­a­ture in de­grees Fahren­heit, among other dif­fer­ences.

ASOS: Au­to­mated sur­face ob­ser­va­tion sys­tems gen­er­ally re­port at hourly in­ter­vals and also re­port spe­cial ob­ser­va­tions if weather con­di­tions change rapidly. They gen­er­ally re­port all the pa­ram­e­ters of the AWOS-3, while also hav­ing the ad­di­tional ca­pa­bil­i­ties.

ASOS pro­vides con­tin­u­ous ob­ser­va­tions nec­es­sary to gen­er­ate a rou­tine weather re­port (metar). They’re more so­phis­ti­cated than AWOS and de­signed to pro­vide the nec­es­sary in­for­ma­tion to gen­er­ate weather fore­casts (TAF). ASOS is com­posed of a stan­dard suite of weather sen­sors.

ASOS can de­ter­mine type and in­ten­sity of pre­cip­i­ta­tion (rain, snow, freez­ing rain), thunderstorms and ob­struc­tions to vis­i­bil­ity such as fog and haze.

ASOS can also mea­sure wind shifts, peak gusts, rapid pres­sure changes and the amount of ac­cu­mu­lated pre­cip­i­ta­tion.

The ASOS sys­tem shines a laser ceilome­ter into the sky to ob­serve cloud lay­ers. It takes a mea­sure­ment every 30 sec­onds over a 30-minute pe­riod, then dou­ble-weighs mea­sure­ments dur­ing the last 10 min­utes so the com­puter can de­cide if the cloud lay­ers are scat­tered, bro­ken or over­cast. Ceil­ing in­for­ma­tion is “time av­er­aged” over a 30-minute

pe­riod, and vis­i­bil­ity over a 10-minute pe­riod.

5. The ASOS vis­i­bil­ity sen­sor is nor­mally lo­cated near the touch­down zone of the pri­mary in­stru­ment run­way.

6. The word “auto” in­cluded in the broad­cast sig­ni­fies that an ob­server is not logged on to the ASOS for backup or aug­men­ta­tion.

7. ASOS in­for­ma­tion can be ob­tained on the proper fre­quency in flight and by tele­phone on the ground. ASOS and AWOS: 1. The flag word “test” is added to the voice trans­mis­sion of an ASOS/AWOS when it is be­ing tested be­fore com­mis­sion­ing and the qual­ity of the ob­ser­va­tions might not be up to FAA stan­dards.

2. The up­per limit of cloud height and vis­i­bil­ity re­ported by ASOS/AWOS is 12,000 feet and 10 miles.

3. The most com­mon rea­son ASOS/AWOS can­not be re­ceived by tele­phone is due to main­te­nance or re­pair.

Food for thought re­gard­ing AWOS/ASOS weather re­ports: The fol­low­ing is an ex­cerpt from an FAA legal in­ter­pre­ta­tion dated May 24, 2012, con­cern­ing VFR flight vis­i­bil­ity re­ported by au­to­mated weather sta­tions.

“You de­scribe a sce­nario where a pi­lot on a VFR flight plan ar­rives at an un­con­trolled air­port in Class E airspace to the sur­face and picks up the AWOS in­for­ma­tion for that air­port. The AWOS re­port states that there is a ceil­ing of 100 feet and vis­i­bil­ity is one-fourth mile with calm winds. You fur­ther ex­plain that the pi­lot flies over the air­port and de­ter­mines that there is a fog bank over one end of the run­way which ob­scures the last 1,000 feet but the other 9,000 feet are clear.

“You asked whether a pi­lot in this sce­nario is in vi­o­la­tion of §91.155 if he lands at the air­port. Yes, if the pi­lot lands at the air­port in the sce­nario de­scribed above, he is in vi­o­la­tion of §91.155. In or­der for a pi­lot to land an air­craft or en­ter the traf­fic pat­tern of an air­port, ground vis­i­bil­ity at the air­port must be at least 3 statute miles. See 14 CFR §91.155(d). The AWOS is re­ported a ceil­ing of 100 feet and vis­i­bil­ity of one-fourth mile. This clearly does not meet the vis­i­bil­ity re­quire­ments as out­lined in § 91.155. The pi­lot’s re­port of flight con­di­tions can­not su­per­sede the AWOS in this sce­nario. The de­ter­mi­na­tion of the vis­i­bil­ity by a pi­lot is not an of­fi­cial weather re­port or an of­fi­cial ground vis­i­bil­ity re­port.”

Note: FAA reg­u­la­tions could change at any time. Please re­fer to cur­rent FARs to en­sure you are legal.

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