Flying - - FRONT PAGE - By Rob Mark

An in­stru­ment land­ing sys­tem (ILS) uses two sep­a­rate ground-based ra­dio trans­mit­ters to pro­vide both lat­eral and ver­ti­cal guid­ance to pilots dur­ing an in­stru­ment ap­proach pro­ce­dure that nor­mally in­cludes the low­est land­ing min­i­mums. A Cat­e­gory I ILS ap­proach, for ex­am­ple, typ­i­cally of­fers a 200-foot de­ci­sion height above the ground. ILS ap­proach vis­i­bil­ity min­i­mums are mea­sured ei­ther in statute miles or by us­ing run­way vis­ual ranges down to ze­rozero for a Cat­e­gory IIIc ILS ap­proach. (Ap­proaches of Cat 2 and greater re­quire spe­cial flight crew cer­ti­fi­ca­tion.)

One ILS trans­mit­ter, called the lo­cal­izer, of­fers

the pi­lot ac­cu­rate left-right guid­ance to a pre­de­ter­mined missed-ap­proach point near the run­way thresh­old. Lo­cal­izer nee­dle move­ments on the cock­pit dis­play are sim­i­lar to what a pi­lot would see nav­i­gat­ing with a VOR and in­clude a ver­ti­cal nee­dle that moves left and right to in­di­cate the po­si­tion of the lo­cal­izer sig­nal. The other trans­mit­ter, known as the glides­lope, pro­vides a direc­tional sig­nal that de­liv­ers pre­cise ver­ti­cal guid­ance. In the cock­pit, the pi­lot will also see an­other nee­dle po­si­tioned hor­i­zon­tally, co-lo­cated with the lo­cal­izer in­di­ca­tor, that moves up and down the face of the in­stru­ment to point to the glides­lope.

Some ILS ap­proaches in­clude dis­tance in­di­ca­tors known as marker bea­cons, la­beled as outer, mid­dle and in­ner mark­ers. The outer marker is nor­mally co-lo­cated with the fi­nal ap­proach fix. Some in­stal­la­tions co-lo­cate a low-fre­quency nondi­rec­tion bea­con with the outer marker to cre­ate a com­plete ap­proach to align an air­craft in­bound on the ILS.

Although the left-right course in­di­ca­tions might look sim­i­lar to those of a VOR, the lo­cal­izer re­ceiver be­comes four times more sen­si­tive when tuned to an ILS. That fact trans­lates into ILS sig­nal pa­ram­e­ters that be­come tighter as the air­craft ap­proaches the end of the run­way, mean­ing the pi­lot must be pre­pared for smaller and smaller cor­rec­tions to re­main on course and on glides­lope. It is the pi­lot’s task to learn the pre­cise speed, rate of de­scent and air­craft con­fig­u­ra­tion needed prior to in­ter­cept­ing the ILS that will al­low the nee­dles to re­main crossed like a huge plus sign, in­di­cat­ing the air­craft is on the lo­cal­izer course and on the glides­lope. For in­stance, at the outer marker, nor­mally 4 to 6 miles from the end of the run­way, a one-dot de­vi­a­tion means the air­craft is about 500 feet off the cen­ter­line. Closer in, such as cross­ing the mid­dle marker within a mile of the end of the run­way, that same one­dot de­flec­tion trans­lates into 150 feet off the cen­ter­line. At the mid­dle marker, one dot high or low on the glides­lope trans­lates into fly­ing 50 feet high or low.

In some lo­ca­tions, the lo­cal­izer emits a sig­nal strong enough to be us­able when ap­proach­ing the air­port from a po­si­tion op­po­site to the front course sig­nal used for a stan­dard ILS ap­proach. For in­stance, the Run­way 1 ILS lo­cal­izer at Chicago Rock­ford In­ter­na­tional trans­mits a solid sig­nal to the north, al­low­ing ar­riv­ing air­craft to use it for lat­eral guid­ance on a back-course Run­way 19 ap­proach. Should a glides­lope for some rea­son fail, ap­proach pro­ce­dures al­low a pi­lot to ex­e­cute a lo­cal­izer-only ap­proach to higher min­i­mums, which trans­forms an ILS from a pre­ci­sion guid­ance sys­tem into a non­preci­sion ap­proach. There is, how­ever, no op­tion for a glides­lope-only ap­proach.

Proper ap­proach light­ing is a nec­es­sary el­e­ment of an ILS and nor­mally in­cludes the run­way’s ap­proach light­ing, as well as touch­down and run­way cen­ter­line lights in ad­di­tion to the stan­dard run­way light­ing.

The cock­pit in­stru­ment in the most ba­sic ILS dis­play in­cludes one ver­ti­cal nee­dle that points to the lo­cal­izer sig­nal, while the hor­i­zon­tal nee­dle rep­re­sents the glides­lope. Some ILS sys­tems in­clude marker bea­cons at spe­cific dis­tances from the end of the run­way that alert the pi­lot with white, blue or yel­low lights and au­di­ble alerts in the cock­pit. Look­ing only at the ILS nee­dles, this in­di­ca­tion shows an air­craft fly­ing well left of the lo­cal­izer course and well above the glides­lope.

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