You can do a lot with an instrument rating and appropriately equipped aircraft—slice through clouds, take-off zero-zero and fly VOR, ILS, GPS and even (if you care) NDB approaches. None of these operations require special qualifications beyond the instrument rating. For example, many of us got instrument ratings before GPS, yet we’re authorized to fly GPS approaches. But, there are things an instrument-rated pilot can’t do, even with certified equipment. Wanna fly a CAT II approach? Sorry. How about an ILS CAT I SA or RNAV (RNP)? Nope, can’t do those either. What gives?
Are You Authorized?
The FAA uses two tools to restrict operations. The first is regulatory. Regulations can specifically prohibit an operation without approval. This is the case of CAT II/III approaches, in which 91.189 states that no PIC can conduct CAT II/III operations without appropriate authorization. Regulatory restrictions also limit the use of RVSM airspace—airspace above FL280 with only 1000 feet between usable flight levels.
Writing regulations is a tedious and arduous procedure for the feds. (See the sidebar.) The other way to restrict an operation is by labeling it “nonstandard.”
Most people, myself included, skim the first paragraph of 91.175. It states that “unless authorized by the FAA,” when it is necessary to conduct an instrument approach, “each person must use a standard instrument approach procedure.” The key word there is “standard.” By classifying a procedure as nonstandard, a procedure requires authorization to fly. Even with appropriately certified equipment, aircraft and even training, these approaches are off limits without that explicit authorization.
In the past, nonstandard approaches weren’t a big deal because they were few and far between. Microwave and transponder landing systems were nonstandard, but few had the equipment to fly those anyway, so it didn’t much matter.
New nonstandard instrument approaches are becoming more common and look deceptively like “standard” approaches. (Tip: You probably could competently fly most of them, but you’re not authorized. In fact, even if you’re authorized when flying for your authorized operator, that authorization typically rests with the operator, not you. So, you might be able to fly an approach at work, but not in your personal twin Cessna, even if suitably equipped.)
The new breed of nonstandard approaches includes ILS (SA CAT I) and RNAV (RNP). These fill an increasing number of sheets in the approach chart booklet. But, are these procedures much different than their standard brethren?
It’s a Special Procedure
Just because an approach is nonstandard doesn’t mean it’s not designed to a standard. All approaches are designed around a thick set of FAA orders that tantalize engineers with diagrams, formulas, and acronyms. Generally, a nonstandard approach requires some capability not associated with a standard procedure. A common example is that an approach might have high minimums due to obstacles on the missed approach segment, but there might be a special approach version with lower minimums that requires a higher-than-standard missed-approach climb gradient. Or, sometimes the FAA is just leery of what an average pilot might do.
An SA CAT I ILS is similar to a standard ILS except it typically requires a heads-up display (HUD) system and often an improved obstacle clearance gradient on the missed. The HUD facilitates the transition to the visual portion of the approach. The approach must be designed with the obstacle clearance gradient for the missed approach starting at the runway, the same as CAT II/III approaches, instead of at DH, which is the case for a normal CAT I. These adjustments to a CAT I approach allow approved operators to drop DH by 50 feet and required visibility by 400 feet.
RNAV (RNP) approaches can be similar to RNAV (GPS) procedures, or vastly different. RNP refers to required navigational performance, or the accuracy of the navigation system. Instead of designating an approach by the system used, RNP designates an approach by navigation capability. Instrument-certified GPS boxes have an RNP of .3. While RNP refers to performance, RNP approaches are designed around different requirements than GPS procedures. RNP approaches incorporate different obstacle clearance requirements, can have radius-to-fix (RF) legs and multiple intermediate segments.
Standard approaches incorporate two obstacle clearance areas; a primary and secondary. Think of the primary area as a box around the flight path. The width and depth depend on the segment of flight. The secondary area is half of the primary, split on either side of the primary, and is a triangle starting at half the depth of the primary. Secondary areas are the “oh no!” buffer between a course and terrain.
RNP approaches don’t have secondary areas to forgive sloppy piloting. Additionally, the width of the primary area is only double the RNP value associated with the approach. With minimums as low as .1 nm, that can put you scary close to some really gnarly obstacles or terrain.
The reduced obstacle clearance areas are useful for areas restricted by terrain or airspace. Another useful feature is radius-to-fix (RF) legs. These are basically DME arcs around imaginary points. Instead of raising an approach to get around obstacles, RF legs can be used to lower intermediate segment altitudes and, ultimately, minimums for approaches with short finals.
Combining RF legs with multiple intermediate segments allows designers to fine tune approaches to match the contours of terrain. The result is the rollercoaster ride printed on the RNAV (RNP) 13 into Palm Springs along a constantly curving path that ends with minimums of 300-1. Palm Springs previously only had circling minimums from a VOR approach with a GPS overlay.
What’s the Big Deal?
OK, SA CAT I and RNP approaches do have a little special sauce. But are they so special a letter of authorization is needed to fly? Generally, no. Many “standard” approaches incorporate special features modifying minimums if using properly certified equipment.
Commonly, ILS minimums can be reduced from twenty-four hundred RVR to eighteen by using a flight director, autopilot, or HUD to DA. (This applies at airports that don’t have the required lighting to support normal eighteen hundred RVR minimums.) If you’re lucky enough to have an enhanced vision system (EFVS), this can be used to establish visibility to descend below DA until one hundred feet above touchdown zone. Using either of these reductions doesn’t require authorization.
If you fly LNAV/VNAV GPS approaches you already fly RNP. According the FAA’s WAAS program manager, WAAS meets the FAA’s requirements for RNP. Additionally, ICAO classifies LNAV, LNAV/VNAV and LPV approaches as RNP approaches.
By the way, flying RF legs isn’t a big deal. A GPS navigator does a much better job flying a DME arc than the turn-and-twist method. I don’t care if the box is turning around a VOR or point in space. Granted, the secondary obstacle clearance area is present for GPS approaches, whether LNAV/VNAV or LPV. However, since these aren’t arcs around a specified fix, they require different math in the navigator and few of today’s navigators in GA use can yet do that math.
The FAA takes a Jekyll and Hyde approach to technology. Excited to embrace the leading edge, the agency pours money into development. But, when it comes to giving up control and letting the masses use it, they’re often slow to give up the keys to the kingdom.
In a sense, classification of a procedure as standard versus nonstandard is arbitrary. The FAA controls the designation. Sources involved with avionics certification believe the FAA is worried about pilot training. The result is that the FAA hesitates to release RNP to the general public. Even the incorporation of RF legs into the LNAV/VNAV obstacle clearance requirements would lower minimums and improve access to many airports.
I Have a Dream
I dream that one day all instrument-rated pilots with a WAAS-enhanced GPS navigator will be able to fly RNP approaches. And if you have a HUD, you will be able to fly CAT I SA. (And, at Christmas time, I still wait up in the hopes of catching a glimpse of the jolly fat man in the red suit coming down my chimney.)
The FAA restricts operations either directly through regulation—as is the case for CAT II/III and RVSM—or through designation of a procedure as nonstandard. ILS CAT I SA and RNAV (RNP) procedures are two common nonstandard instrument approaches.
An ILS CAT I SA incorporates an improved obstacle clearance area for the missed approach and requires the use of a heads-up guidance system. In exchange, DH drops by 50 feet and RVR by 400 feet.
While a heads-up system is out of financial reach for most piston flyers, many already have the equipment to fly RNP approaches. The ADS-B requirements can offer this capability by 2020. Yet, RNP approaches remain nonstandard due to RF legs, reduced obstacle clearance areas and training worries.
Maybe in the future the status of these procedures will change. Meanwhile, at least you know why you can’t fly ’em.
As a pilot for a major airline, Jordan Miller has access to many of these approaches—but only at work. On his own, he’s limited like the rest of us.