GO IN­SIDE YOUR MODE C TRANSPON­DER

ALL ABOUT MODE C AL­TI­TUDE RE­PORT­ING TRANSPON­DERS

Flying - - FRONT PAGE - By Richie Len­gel

REL­E­VANT DIS­CUS­SION:

91.215, 91.217, 91.135, 91.411, 91.413, 99.13, 121.345, 135.143, AIM 3-2-3, 3-2-4, 4-1-20, 5-6-4, 6-2-2, 6-4-2, FAA-H-8083-16, 8083-25

• Re­quired for all air­craft in Class A, B and C airspace.

• Re­quired for all air­craft in all airspace within 30 nm of an air­port listed in ap­pen­dix D, section 1 of Part 91 (Class B and mil­i­tary) from the sur­face up­ward to 10,000 feet msl.

• Not­with­stand­ing the 30 nm rule, any air­craft that was not orig­i­nally cer­ti­fied with an en­gine-driven elec­tri­cal sys­tem or that has not sub­se­quently been cer­ti­fied with one, or a bal­loon or glider, may con­duct op­er­a­tions in the airspace within 30 nm of an air­port listed in ap­pen­dix D, section 1 of Part 91, pro­vided such op­er­a­tions are con­ducted out­side any Class A, B or C airspace area be­low the al­ti­tude of the ceil­ing of a Class B or Class C airspace area des­ig­nated for an air­port or 10,000 feet msl, which­ever is lower.

• Re­quired for all air­craft in all airspace above the ceil­ing and within the lat­eral boundaries of a Class B or Class C airspace area des­ig­nated for an air­port up­ward to 10,000 feet msl.

• Re­quired for all air­craft (ex­cept any air­craft that was not orig­i­nally cer­ti­fied with an

en­gine-driven elec­tri­cal sys­tem or that has not sub­se­quently been cer­ti­fied with such a sys­tem in­stalled, or a bal­loon or glider) in all airspace of the 48 con­tigu­ous states and the Dis­trict of Columbia at and above 10,000 feet msl, ex­clud­ing the airspace at and be­low 2,500 feet above the sur­face. [Note: §91.215(b)(5)(ii) also re­quires Mode C from the sur­face to 10,000 feet msl within a 10 nm ra­dius of any air­port listed in ap­pen­dix D, section 2 of Part 91. How­ever, ap­pen­dix D, section 2 of Part 91 is “re­served,” and there are no air­ports listed.]

• Com­mon squawk codes

1200 — VFR

1202 — Glider

1255 — Fire­fight­ing

1277 — SAR

7700 — Emer­gency

7600 — Lost com­mu­ni­ca­tions 7500 — Hi­jack

7777 — Mil­i­tary in­ter­cep­tor 0000 — Mil­i­tary op­er­a­tions

• The Mode C trans­mit­ted must agree with the baro­met­ric al­time­ter within ±125 feet. [91.217(b)]

• The transpon­der, in­clud­ing Mode C and

ADS-B Out, shall be op­er­ated at all times in all con­trolled airspace and all airspace spec­i­fied in 91.215(b) un­less oth­er­wise au­tho­rized or di­rected by ATC. They should also be turned “on” (in­clud­ing Mode C and ADS-B Out) prior to move­ment on the air­port sur­face. (AIM 4-1-20)

• An air­craft with an in­op­er­a­tive transpon­der may be flown to the air­port of ul­ti­mate des­ti­na­tion, in­clud­ing any in­ter­me­di­ate stops, or pro­ceed to a place where suit­able re­pairs can be made, or both. The re­quest to do this can be made at any time. [91.215(d)]

• For op­er­a­tion in any airspace listed above for an air­craft not equipped with a transpon­der, the re­quest to op­er­ate must be made at least one hour be­fore the pro­posed time. [91.215(d)]

• Mode C al­ti­tude trans­mis­sions are

in­de­pen­dent of the baro­met­ric al­time­ter.

The transpon­der can get its in­for­ma­tion from one of two sources: an en­cod­ing al­time­ter, which trans­mits a pres­sure al­ti­tude read­ing to the transpon­der, or — more com­monly — a blind en­coder, an al­time­ter with­out nee­dles or ad­just­ment knob per­ma­nently set to 29.92 (pres­sure al­ti­tude). In ei­ther case,

the al­time­ter set­ting does not af­fect the al­ti­tude the transpon­der sends. ATC’s com­put­ers ap­ply the cur­rent al­time­ter set­ting to the pres­sure al­ti­tude re­ceived, con­vert­ing it to msl (which should match your in­di­cated al­ti­tude). There­fore, you can­not fool ATC by re­set­ting your al­time­ter (of course, you could al­ways fly the air­plane a lit­tle higher or lower — that’ll fool ’em!). ATC will most likely ask you to “stop alti

tude squawk” if your in­di­cated al­ti­tude and the al­ti­tude re­ceived by ATC dif­fer by 300

feet or more. The transpon­der trans­mits pres­sure al­ti­tude to ATC in 100-foot in­cre­ments.

• Ident but­ton — Press­ing the ident but­ton

(“squawk ident”) sends an ex­tra pulse to ATC that causes your tar­get to flash on the con­troller’s radar scope to help lo­cate or ver­ify your tar­get. This should cause the re­ply light on the transpon­der to stay full bright for a few sec­onds, af­ter which it re­sumes its nor­mal spas­modic flash­ing as it is in­ter­ro­gated. Ident also sends a stronger sig­nal use­ful in ar­eas of weak radar cov­er­age. If the con­troller asks you to ident, the best re­sponse is sim­ply to state your tail num­ber as you push the ident but­ton. This but­ton should never be pressed un­less re­quested by ATC. It can also be used for

com­mu­ni­ca­tion, that is, to al­low a pi­lot to re­spond if their ra­dio trans­mit­ter has failed. If ATC thinks there’s a pos­si­bil­ity you might be able to hear them, but not have the abil­ity to trans­mit a re­ply, they may ask you to “squawk ident.” “Ci­ta­tion 321GO, if you can hear cen­ter, squawk ident.” Fur­ther in­struc­tions can then be ac­knowl­edged by press­ing ident again.

• For flight in con­trolled airspace un­der IFR, al­ti­tude en­coders must be tested and re-cer­ti­fied ev­ery two cal­en­dar years. [91.411(a)]

• En­coders typ­i­cally use a lot of elec­tri­cal power. In a sit­u­a­tion where you lose an al­ter­na­tor, you might strongly con­sider turn­ing off the Mode C func­tion and no­ti­fy­ing ATC.

• MODE S is re­quired for TCAS II op­er­a­tion. Mode S has the same ca­pa­bil­i­ties as Mode A and Mode C, and also re­sponds to traf­fic alert and col­li­sion avoid­ance sys­tem

(TCAS)-equipped air­craft. It pro­vides the airto-air datalink be­tween TCAS II-equipped air­craft to co­or­di­nate res­o­lu­tion ma­neu­vers. This en­sures that the res­o­lu­tion ad­vi­sory dis­played in one TCAS II-equipped air­craft

is com­pat­i­ble with the ma­neu­ver dis­played in the other TCAS II-equipped air­craft.

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