THE “HUNDRED ABOVE”
When flying an approach in marginal weather, it should be obvious that the flying pilot is looking outside for any sign of the runway. That’s all good and well, but he may miss the altitude he’s passing in the meantime, which requires a glance at his instruments inside the cockpit.
Thankfully, the airplane calls the minimum altitude for the approach out loud for the pilots. Not only does the airplane take care of that, it also has a nice automatic call to alert them that those minima are going to be reached damn soon. That’s the “hundred above” call.
INTRODUCING MINIMA
Airports, especially those visited by international airliners, have specific approach procedures published. These procedures are designed to bring airplanes in on a path towards an otherwise possibly hard to find runway.
Especially when weather is marginal, if there’s a low cloud base or visibility is lousy, pilots would never find a runway unless there’s such a published approach procedure.
Below is an image I found of two different approach types to the same runway at the same airport.
(The left chart is an ILS approach - a type of precision approach. An airplane flying this approach gets guidance laterally and vertically by a signal sent from the airport. Note the numbers on the bottom of the chart, below the vertical profile, below the A, B, C and D columns: 282/24. It’s the 282 that matters for this answer. The right chart is an RNAV (GPS) approach - a type of non-precision approach. It gives lateral guidance by means of GPS, but there’s no signal telling pilots how they’re doing vertically. The only thing they know is that by certain positions they should have a certain altitude (like 3400 ft at IVUVY). Again there are numbers under the A, B, C and D columns. The 440 is the number that matters here in this answer, for this approach.)
These are the altitudes by which a pilot must see the runway to be able to continue and land. If there’s not enough visual reference of the touchdown zone or runway by that altitude, the approach must be discontinued and a go around or missed approach flown. If no improvement is expected, this may lead to a diversion to another airport.
Clearly, if possible, when weather isn’t great, it’s better to fly the precision approach on the left, as it give more chance to see the runway at the lower 282 feet than at the 440 feet of the non-precision approach.
These 282 ft and 440 ft are called the “MINIMA” or minimum altitudes. (The heights are between brackets on the charts - in aviation we distinguish between height and altitude, and even level and elevation. They all mean different things.)
Side note: depending on the approach types, these minima also go by the name of “decision altitude/height” (DH), or “minimum descend altitude” (MDA). A small confusion may arise here because Airbus uses these terms to distinguish between barometric altitude and radio altitude, while per procedural terms they define whether it’s a precision or non-precision approach.
So for the approach on the left, the “hundred above” call would come at 382 feet. When pilots fly the approach on the right, the “hundred above” call would come at 540 feet. In fact, it also depends on them programming those minimum altitudes correctly in their airplane’s systems, of course.
When flying an approach in marginal weather, it should be obvious that the flying pilot is looking outside for any sign of the runway. That’s all good and well, but he may miss the altitude he’s passing in the meantime, which requires a glance at his instruments inside the cockpit.
Thankfully, the airplane calls the minimum altitude for the approach out loud for the pilots. Not only does the airplane take care of that, it also has a nice automatic call to alert them that those minima are going to be reached damn soon. That’s the “hundred above” call.
INTRODUCING MINIMA
Airports, especially those visited by international airliners, have specific approach procedures published. These procedures are designed to bring airplanes in on a path towards an otherwise possibly hard to find runway.
Especially when weather is marginal, if there’s a low cloud base or visibility is lousy, pilots would never find a runway unless there’s such a published approach procedure.
Below is an image I found of two different approach types to the same runway at the same airport.
(The left chart is an ILS approach - a type of precision approach. An airplane flying this approach gets guidance laterally and vertically by a signal sent from the airport. Note the numbers on the bottom of the chart, below the vertical profile, below the A, B, C and D columns: 282/24. It’s the 282 that matters for this answer. The right chart is an RNAV (GPS) approach - a type of non-precision approach. It gives lateral guidance by means of GPS, but there’s no signal telling pilots how they’re doing vertically. The only thing they know is that by certain positions they should have a certain altitude (like 3400 ft at IVUVY). Again there are numbers under the A, B, C and D columns. The 440 is the number that matters here in this answer, for this approach.)
These are the altitudes by which a pilot must see the runway to be able to continue and land. If there’s not enough visual reference of the touchdown zone or runway by that altitude, the approach must be discontinued and a go around or missed approach flown. If no improvement is expected, this may lead to a diversion to another airport.
Clearly, if possible, when weather isn’t great, it’s better to fly the precision approach on the left, as it give more chance to see the runway at the lower 282 feet than at the 440 feet of the non-precision approach.
These 282 ft and 440 ft are called the “MINIMA” or minimum altitudes. (The heights are between brackets on the charts - in aviation we distinguish between height and altitude, and even level and elevation. They all mean different things.)
Side note: depending on the approach types, these minima also go by the name of “decision altitude/height” (DH), or “minimum descend altitude” (MDA). A small confusion may arise here because Airbus uses these terms to distinguish between barometric altitude and radio altitude, while per procedural terms they define whether it’s a precision or non-precision approach.
So for the approach on the left, the “hundred above” call would come at 382 feet. When pilots fly the approach on the right, the “hundred above” call would come at 540 feet. In fact, it also depends on them programming those minimum altitudes correctly in their airplane’s systems, of course.
