SVS, huh? What is it good for?

“I don’t fly in the mountains, so I really don’t need synthetic vision.”

If I had a dollar…Yet I can’t blame the average pilot who has yet to experience SV for sharing this opinion.  The marketing materials of avionics manufactures are full of scary pictures of red and yellow mountain tops depicted on the PFD.  That or a low pass over a big cell tower.  Presented thus, it’s easy to see synthetic vision (SV) as solely a tool to avoid hitting things.  Yet even if you fly in the flatlands, SV is one heck of a cool feature to have.  Here’s why.

Let’s start with a tour of some recent avionics innovations.  When the first IFR GPS units were certified I remember flying an approach and thinking “This is great- every approach can now be flown like a Localizer approach.”  Flash forward a few years to WAAS- now every approach can be flown like an ILS, even better!  Well- what’s even easier than an ILS? A visual, of course, and SV had moved the state of the art to where every approach can encompass the same instant situation awareness experienced on a visual.  Of course, the published procedure must still be followed, but seeing the runway symbol and instantaneous flight path marker (FPM) crisply drawn on the PFD means the same intuitive techniques used to line up with a runway on a clear VMC day can be used when it’s 200-and-a-half.

For example, imagine a descent is being made to MDA on a non-precision approach.  No vertical guidance is provided, so the pilot has no choice but to dive to MDA, right? Not with SV- by controlling rate of descent so that the runway symbol stays at the minus 3° pitch point of the PFD, the plane is descending as smoothly on a 3° path as if on a glideslope.

Caption: Runway at three degrees and flight path marker on runway= stable approach

Another time SV can be a great aid to stabilized approaches is when cleared for a visual approach to an unfamiliar airport from a base leg entry.  These can be difficult to plan correctly, as the visual cues are difficult to interpret at a ninety degree angle to the runway.  Often pilots will turn final and find themselves quite a bit higher than anticipated.

If the runway has an approach with vertical guidance published, rolling out right on glidepath is child’s play.  By loading the approach and activating the leg to the runway threshold, the pathway boxes will be visible in profile.  They will be descending to the runway on roughly a 3° angle, so steering the FPM so that it lies over any pathway box will ensure the aircraft is descending so as to perfectly intercept the final on glidepath.

Caption: “Gunsighting” pathway box with FPM to ensure rolling out on glidepath

Y or Z, Part II

Last post I looked at the situation where a non-standard missed approach gradient could cause two sets of minimums to be needed for the same approach, thus resulting in two versions of the approach, a “Y” and a “Z”.  I mentioned there are three reasons two approaches of the same type could exist to the same runway, let’s look at the other two now- both involve RNAV approaches.

The second case is where two RNAV (GPS) approaches exist to the same runway.  Typical of this example is Burlington, VT, with an RNAV (GPS) Y and Z approach to runway 15.  The Z approach has published WAAS (or FMS) minimums of LNAV/VNAV to 660’ and LNAV-only mins of 940’.  The Y approach only has LNAV mins published, but to 760’, lower than the LNAV mins for the Z approach.   A quick view of the profile view shows why; the Y approach has a stepdown waypoint, JUNEL, abeam a 639’ high tower.  So an aircraft not equipped with WAAS/ FMS would want to execute the Y approach, one capable of LNAV/VNAV approaches would want the Z approach.

BTV RNAV (GPS) Y RWY 15

 BTV RNAV (GPS) Z RWY 15

Sometimes the approach course is completely different, as is the case with the RNAV (GPS) Y or Z --- at Half Moon Bay, CA.  The Y approach has LNAV only minimums, and is flown mostly over the water, with two doglegs, or course changes.  The Z approach takes advantage of LPV’s tighter guidance to get down twice as low as the Y, and is aligned with the runway from a 14 mile final.

HAF RNAV (GPS) Y RWY 30

 HAF RNAV (GPS) Z RWY 30

The other case where multiple RNAV approaches are published to the same runway involves a new type of approach- the required navigation performance, or RNP approach.  RNP approaches are the approach of the future, and represent a shift from navaid- based approaches to performance-based approaches.  In other words, as opposed to a VOR approach, which can only be flown by reference to the designated VOR, an RNP approach only requires that an aircraft be able to maintain a specified level of accuracy and integrity.  For all practical purposes, RNP approaches now require GPS, but the option in the future to rely on other sensors exists.

An RNP approach is an RNAV approach in that it depends on the ability to navigate to any arbitrary point in space.  Yet its equipment requirements go beyond an IFR GPS unit, so it is classified RNAV (RNP), as opposed to RNAV (GPS).  A way is needed to differentiate between these multiple RNAV approaches, so again X, Y, and Z come into play.  An example is San Francisco, with an RNAV (GPS) Z RWY 28R, and an RNAV (RNP) Y RWY 28R approach.

We’ll look more at RNP approaches in the future, but for now there aren’t many GA aircraft capable of flying them.  Only the flagship products from the largest business jet manufactures have received approval, and individual operators and pilots need authorization to fly even an approved aircraft on these approaches.

Y or Z?

A CJ2 flight into Jackson Hole, WY brought up an issue that confuses many instrument pilots- what does an X, Y or Z in the approach title signify?  Examples are the KJAC ILS or LOC Z Rwy 19, or the KSFO RNAV (GPS) Y Rwy 19L.

The answer is fairly simple, but like many answers, it only begs further questions be asked.  When there exists more than one approach of a given type to a specific runway, the approaches will be titled uniquely by including Z, Y, X, etc… in the title.  Thus the presence of an RNAV X Rwy 9 approach at an airport implies there are at least three RNAV approaches to runway 9.  If only two exist, they will be Z and Y; the convention starts at Z and works backwards into the alphabet.  Letters at the end of the alphabet are used so there is no confusion with the labeling for circling-only approaches, which are named with letters starting with A, moving into the alphabet  (The first circling-only VOR approach at an airport is titled VOR-A, the next is VOR-B, etc…).

So why would there be more than one approach of a given type to the same runway? Wouldn’t the FAA just make one approach as “good” as possible?  I’ve identified three broad reasons this isn’t always a viable option- for this post we’ll look at the reason Jackson has two ILSs.

Here, the difference between the Y and Z can be subtle to spot.  Looking at the profile and plan view of the charts, the Y and Z ILS seem to be identical procedures.  A careful look shows the ILS minimums are different, though. The Z approach brings a plane down to 200’ above TDZE, and requires ¾ mile visibility, while the Y only goes down to 612’ above TDZE, and needs 2 miles visibility.

So why would anyone shoot the Y? The answer lies in the notes.  On the Z approach is stated “Missed approach obstructions require a minimum climb gradient of 245 feet per NM to 11200; if unable to meet rate of climb, see ILS or LOC Y RWY 19.”

The standard gradient for a climb that is part of an IFR procedure is 200’ per NM.  If this gradient will not ensure adequate separation from terrain, a higher gradient may be required.  This is commonly seen on departure procedures, but as an aircraft starts a missed approach from a few hundred feet up, and from a point often before the runway starts, it’s uncommon to see a missed approach that requires a non-standard gradient.

 

By starting the missed approach 412’ higher, and almost 1.5 miles further from the runway, an aircraft conducting the Y approach will maintain adequate separation even if only climbing 200’/ NM.

Don't B too Fast

At first, I was surprised reading a recently published article which stated that exceeding the "200 KIAS Under Class B" limit is the most common violation for turbine pilots.  But really I shouldn't have been, as the rule has two great ingredients to bake up a violation:

1. It's easy for ATC to know you've violated it, and

2. It's easy to violate.

Both departures and arrivals can create great chances for this particular bust.  Taking off from BED, Mass, for example, aircraft are invariable cleared to a 2,000' initial altitude.  Depending on the direction of take off, the floor of the BOS Class B lies at either 3,000' or 4,000', so an aircraft leveling at 2,000' must remain at or below 200 KIAS even when out of the Class D speed restriction area (4 NM).  This can be especially difficult for a lightly loaded and/or very high performing airplane.  The pilot must contend with not only a quick level off shortly after gear and flap retraction, but must simultaneously keep airspeed down.

One technique than can help in this situation is use a thrust commanded climb once cleaned up.  Set the FD/ AP to an indicated airspeed mode slightly below 200 KIAS, and reaching 1,000' before the level-off reduce thrust as necessary to maintain an approximate 1000 FPM climb rate.  As the aircraft levels off it will be significantly easier to keep speed in check than had the climb been at full power.

Arrivals can create a more vexing situation where simply knowing when the aircraft is under Class B airspace can be difficult.  This is especially true in aircraft with legacy avionics lacking a moving map or with a basic map lacking Class B boundaries.  But even new aircraft with a G1000 based system display only the lateral limits of Bravo airspace on the MFD; determining where the floor is requires active button pushing and knob turning.

As individual sectors can be mere miles across, a pilot can be faced with determining the floor of several sections in quick succession, while simultaneously configuring the plane for landing, receiving rapid heading and altitude changes, and monitoring ATC in what's obviously a busy environment (or it wouldn't be Bravo to begin with).

To add to the fun, ATC can't authorize a speed over 200 KIAS below the Bravo- if assigned 230 KIAS, for example, it's incumbent upon the pilot to slow to 200 if altitude clearances will take the aircraft out of the Bravo.  FAR 91.117(c), which spells out the Bravo speed restriction, doesn’t contain the verbiage “…unless otherwise authorized or required by ATC…”, which is included in the restriction on Class C and D airspeed.

Heads up out there!