As an instrument instructor I’ve learned one of the most common topics of confusion among pilots is, “When is it safe—and expected—to descend in the process of an instrument arrival?” Confusion isn’t limited to non-professionals. One of the most frequent topics for quizzes and reviews in professional pilot journals involves determining the correct altitude to fly at various points on a charted terminal procedure.
Why is the question of “letting yourself down” so challenging? What are the basic rules about when it’s safe to descend into and on an approach?
From En Route to Arrival
In general, you should have a minimum altitude firmly in mind before you begin your descent from cruise altitude. Most of the time this is simple—descend to the altitude to which you have been cleared.
Caution is prudent, however, so before you begin your let-down, take a quick look at the minimum safe altitude for your current location. This could be a minimum en route altitude (MEA) if you’re on an airway, are using ground-based navaids defining the route (almost always VOR) and are more than 25 nm from the navaid defining your current segment of the route. If you’re within 25 nm or you’re navigating along the airway using GPS, you can safely descend to the MOCA—the minimum obstacle clearance altitude.
If you’re off-airways, either flying GPS or on vectors from ATC, the lowest safe altitude is the off-route obstruction clearance altitude (OROCA) for the area in which you’re flying (see the chart excerpts on page 14). All of these altitudes provide at least 1000-foot clearance above terrain or man-made obstacles in the depicted area, and 2000 feet in mountainous areas. The MEA also guarantees ground-based navaid reception along the entire airway.
Shouldn’t you just descend to the altitude to which you’ve been cleared? Not without crosschecking against the lowest safe altitude. More than once I’ve been cleared to an altitude lower than the charted minimum safe altitude for the area. I’ve queried ATC and, most of the time, confirmed they have me on radar and are able to descend me a little lower in the specific area I’ll fly. Controllers are human, too, and a clearance today would not be the first time a flight was under positive control all the way into terrain because the controller made a mistake and the pilot blindly followed the clearance.
As FAR 91.123 states, upon receipt of a clearance the pilot in command has the authority to accept a clearance or reject it and request a change. Reading between the lines, this means the PIC must decide whether the clearance meets his/her needs and may be safely flown. That decision, in turn, requires the pilot to compare the clearance to obstacle avoidance requirements.
The takeaway? Use your charts (paper or electronic) to determine the lowest safe altitude before you accept a clearance to descend even lower.
STARs and ARCs
The STAR, or standard terminal arrival, removes some of the guesswork of letting down toward an instrument approach. Although some STARs are fairly complex (see the chart excerpt on page 14), once you know the routing you’ll fly from the en route to the terminal area, you can determine the minimum altitudes to expect on each segment of the procedure. Although ATC may assign higher altitudes for some reason, the recommended altitudes give you an idea of what to expect as a minimum.
Meanwhile, the DME arc is one of those procedures you either use a lot or almost never. In some parts of the country, they are common—the arc is a convenient way for pilots to “self-vector” for an approach with a big reduction in controller workload, so they are popular in high-traffic areas. Basically, the DME arc is a simple STAR made more complicated only in that it involves a continuous turn until the lead-in to intercept the final approach course. Like a STAR, DME arcs include a recommended altitude to be flown.
Flying On Vectors
Once, I questioned a clearance to descend to 3000 feet while on vectors to an approach into the Kansas City Downtown Airport because the charted procedure called for an initial altitude of 3200 feet. The controller responded, very apologetically, that my clearance was amended to maintain 3200 feet. As I was being vectored to intercept the final approach course I directly overflew a tall tower, probably the reason for the 3200-feet restriction.
If I had not queried the clearance based on looking at the approach I was being vectored to fly, and the weather was IMC or it was night, if I had adjusted my altimeter setting incorrectly (the altimeter itself was at the extreme of its allowable tolerance) and I was just a little below my assigned altitude (an unusual but not impossible confluence of conditions), I might have run into the tower while under positive air traffic control.
Another aid for altitude awareness on vectors is the minimum sector altitude (MSA) depicted on approach charts (see the chart excerpts above). The MSA is the minimum obstacle clearance altitude surrounding identified fixes (and not always the airport or the final approach fix, so watch out) and is further defined by radials or bearings and distances from that fix. The MSA’s intent is to give the pilot a minimum altitude for emergencies—e.g., ice accumulation or partial power loss that prevents or inhibits climb capability—combined with some factor making it necessary to deviate from a charted procedure or impossible for the pilot to comply. In other words, you never want to have to use the MSA, but you want to know what it is.
Note that, on defined ground tracks either following a charted procedure or radar identification by controllers, you often will be assigned an altitude to intercept the approach that is below the MSA applying to that entire sector.
Inbound On The Approach
All through your arrival, you’re waiting for the magic words “cleared for the approach” (controllers must use that precise phrase). Once you’re cleared and aligned on a published segment of the procedure, you are responsible for flying the altitudes charted for each segment of the approach. In most cases, the altitudes are recommended (not mandatory), but they are designed to permit you to arrive with a minimum of steep descents, to give you time to visually acquire the runway environment and maneuver as necessary to land. In other words, fly the charted altitudes.
The most common area of confusion I see among pilots flying a vectored or straight-in (no procedure turn) final approach course is determining when it’s safe to descend to the initial or intercept altitude. What if you intercept the final approach course 20 miles out? Unless noted otherwise on the approach chart, you should be within 10 nm of the FAF and aligned on the final approach course before descending to the depicted altitude once cleared for a long, straight-in approach.
Flying The Full Procedure
Another frequent IFR pilot mistake is to descend too soon during a procedure turn. Most of us do not fly a lot of procedure turns or holding pattern-style course reversals in our everyday flying, so we don’t have the depth of experience in the maneuver that we do in other arrival procedures.
Basically, fly the altitude you have been directed to hold until “established.” Once you cross the holding fix outbound, descend to the altitude depicted for the hold or procedure turn. As you turn inbound and the course needle indicates you are intercepting the final approach course, then you can descend if needed to the altitude shown for the final approach course inbound to the final approach fix (FAF).
Once you pass the FAF inbound, fly to the decision height (DH), decision altitude (DA) or minimum descent altitude (MDA) for the approach. There may be one or more step-down fixes between the FAF and the missed approach point (MAP). These are designed to give you vertical clearance over an obstacle until you can determine you have passed that obstacle, after which you may descend lower.
If there is a step-down fix on the approach chart, you may not descend below the altitude depicted for each step-down segment. For example, when inbound on the RNAV RWY 2 approach to Idabell, Okla., excerpts from which are reproduced on the opposite page, and after crossing the FAF, you may descend to 2100 feet until passing WOSAT, after which you may descend to the MDA.
Once you have the runway environment in sight, of course, you may descend below DH, DA or MDA as appropriate to the approach you’re flying…with two exceptions.
Visual Descent Point
The first exception is when the approach has a charted visual descent point (VDP). The VDP is a defined point on the final approach course of a non-precision, straight-in procedure. At the VDP, you may commence a normal descent from the MDA to the runway touchdown point, provided you have the necessary visual reference. A common instrument pilot error is to descend from MDA as soon as the runway environment is in sight, putting you in potential conflict with towers, wires, trees or other hard-to-see-in-IMC-or-at-night obstacles the charted VDP is designed to help you avoid.
Instrument runways very often have some version of a visual approach path indicator. A good technique is to maintain approach guidance until reaching the MAP, then intercept and follow the VASI, PAPI or other visual indicator to the touchdown point. Sometimes visual indicators are set for a steeper than normal approach to compensate for obstacles below DH, DA or MDA.
Descending While Circling
The second exception is when flying a circling approach. In a circle-to-land maneuver, you must maintain circling minimums until you are in a position to make a normal rate of descent to the touchdown point.
With most approaches, circling minima are 400 to 600 feet above ground level. Think for a moment about where in the pattern you’d normally be at 400 to 600 feet agl. Generally, this is short final. That’s why, although it’s not in the regulations, many pilots are taught to maintain circling minimums until the airplane is aligned with the final approach course (some instructors teach when within 45 degrees for the final approach heading). On an approach with very high circling minima, perhaps in mountainous terrain, you’d need to descend sooner once the airport is in sight. But in most cases the 45-degrees-of-runway-heading advice is valid.
What about altitude control when flying a visual approach? Whether you cancel to descend or you’re cleared for a visual approach, you now assume all responsibility for terrain and obstacle clearance. All the guidance above about charted altitudes applies—because your life depends on knowing the lowest safe altitudes.
Missing
Many pilots equate landing out of an approach with “success” and missing an approach with “failure.” One of the most tempting situations is when you begin a missed approach and then break out of the clouds with the runway in sight. Every nerve in your goal-oriented body say “chop the power, dump the flaps and land.” This is almost never a good idea, as the NTSB record attests.
Once you determine the need to miss an approach, fly the missed. Don’t try to radically change from a power-up, clean-up, climb-up mentality to one embracing landing at any cost. You may stall, you may descend into terrain or you may land fast and long, and go off the far end of the runway. None of these outcomes is worth the risk of trying to deviate from your decision to miss.
Letting Yourself Down
My experience as an instrument instructor is that many pilots don’t take the time to consider the minimum safe altitude for each segment of a flight. They tend to take ATC directions without question (controllers have human factors too) and don’t stay in the books enough to know precisely when it’s safe to descend when flying procedures they don’t normally use.
It’s a great, big world down there beneath the clouds, and you don’t want to run into it. Use all the information on your charts to safely let yourself down.
Tom Turner is a CFII-MEI who frequently writes and lectures on aviation safety.
