Monday, August 01, 2005

Precision and Non-Precision

I've been avoiding stating the difference between precision and non-precision approaches as I discuss minima because it's too long for a parenthetical remark, so now I'll give it its own post. They are two classes of instrument approach procedures, based on the equipment available in the vicinity of the airport, and in the aircraft. The same airport, indeed the same runway, may have both a precision and a non-precision approach. I believe all Canadian precision approaches include a corresponding non-precision approach, but I haven't searched all my approach plates to make sure.

A precision approach provides both horizontal and vertical guidance to the runway. In other words, it's tells you if you're deviating to the left or right and it tells you if you're too high or too low, before you ever see the runway or any of the ten items associated with the runway visual environment. By far the most common type of precision approach, and really the only one available to civilians is the ILS. (It stands for Instrument Landing System, but we only ever say "eye-ell-ess.") Imagine a beam that extends from the desired touchdown point on the runway back up the approach, and imagine the pilot using instruments in the cockpit to keep the aircraft centred in the beam. That's not quite how the ILS works, but it's an adequate model. If you haven't found the runway by the time you reach decision height, usually 200' agl, you fly the missed.

A non-precision (NP) approach gives only horizontal guidance. That is, it tells you if you are off to the left or the right, but not how far above or below the glidepath you are. Of course it has to give some vertical guidance, which it does in the form of telling what altitude to start at, and what altitude to be at by each of a number of points, sometimes specified by DME (distance measuring equipment -- yes, aviation has some of the stupidest abbreviations) or passing overhead a radio beacon called an NDB (non-directional beacon). The lowest altitude you may descend to before sighting the airport is called the minimum descent altitude, or MDA. You stay at the MDA until you either see the runway or reach the missed approach point (MAP). If you see the runway, and it's possible to land safely from where you are, then you do so, otherwise you fly the missed.

One tricky thing about a NP approach is that the MAP may not be a specified point like "the beacon that broadcasts 'HI' in Morse code." It might be an amount of time past the beacon that broadcasts 'HI' in Morse cose, determined by how fast you're flying through the air, the wind strength and direction, and a little table in the corner of the approach plate. I think that sounds worse than it is, so I'll show an example.

I have here an expired approach plate for the ILS or NDB RWY 09 approach to the John G. Diefenbaker International Airport in Saskatoon, Saskatchewan. (Sorry, I don't have a scanner, and I don't know where to find it online). We'll imagine the pilot doesn't have an ILS receiver in her airplane, perfectly legal in Canada, so she'll be doing the NDB approach. The diagram shows the XE NDB (i.e. the beacon that broadcasts Morse -..- . in all directions) aligned with runway 09 and located on the ground 3.6 miles before the runway threshhold. According to the plate, once the pilot is cleared for the approach, and is within 100 nm of the airport, she may descend1 to 4500' asl. Within 25 nm of the XE NDB she may step down to either 3300', if she's coming from the west, or 3700', if she's coming from the east. Once she has reached the beacon, turned around, and established herself on the horizontal guidance of the approach, coming straight towards the beacon on a track of 087 degrees, she may descend to 3000'. All these numbers are on the approach plate, with little coded diagrams. As she passes overhead the beacon, she may begin descent to the MDA of 2020 feet (367 above the ground, according to the plate). If she had DME she'd probably use that to find the MAP, but if her employer would pay for DME, she'd probably have a functional ILS, too, so she has to use the little table.


She knows her true air speed for the approach is 120 knots, and we'll say she knows she is facing a fifteen knot headwind, so her groundspeed is 105 knots. That's faster than 90 knots, but slower than 110. From the table there's a 26 second spread over that 20 knot range, and five knots is a quarter of that, so six or seven seconds longer than the 110 knot case: giving a timing of 2:05. The pilot works this all out before starting the approach. On reaching the beacon she will start her stopwatch, reduce the power, and descend promptly to 2020 feet. For the best chance of landing, she must get there before her two minutes and five seconds have elapsed, because MDA or not, when the time is up, she's leaving. Most NP approaches are designed such that if you see the runway at MDA just as the time elapses you still have to go missed because they bring you out right above the threshold, too high to land. The trick is to chop and drop so that you are established at the MDA in time to see the runway.

This is where you want to look at the advisory visibility and calculate when you actually have to see the runway. The advisory visibility for the approach above is one mile. At 105 knots, she covers a mile in about 34 seconds, so she needs to be at MDA within 1:31 of the beacon to make this approach. When she crosses the beacon a 700 foot per minute descent rate should be enough to make that happen.

If the cloud bases are at 1900', no matter how fast she gets down and no matter how good the visibility underneath is, she's not going to see anything at 2020', and is really going to be wishing she had an ILS, because the DH for the ILS is 1853'2 and she'd be going home instead of heading to a motel in Lloydminster.

Technically, only ILS approaches have a DH. NP approaches may be based on a variety of navigational aids, but they all have an MDA and may have a time. GPS approaches for the time being are considered NP, but that may change.


1. I understand that in the United States she would need a separate descent clearance, but in Canada "cleared for an approach" allows her to descend.
2. Yes, 1853'. Not 1854' and definitely not 1852'. I may do another post on how they come up with these numbers.


Avimentor said...

In the US, being cleared for an approach allows you to descend, unless the controller provides some other restriction. The usual format is "Barnburner 135 is six miles from RORAY, fly heading 250, maintain 3000' until established, cleared for the NDB 27 Right approach." Of course, many of our NDBs are being decommissioned ...

GPS approaches that provide vertical guidance (the new LPV - localizer precision with vertical guidance) also provide a Decision Height - usually 250' HAT. What is the status of WAAS in Canada, BTW?

Anonymous said...

In the U.S., does an approach clearance allow you to descend before you enter the approach?

For example, if you're at 4,000 ft and cleared for an approach 20 nm back from the airport (with no other instructions), can you start descending immediately? I had understood that you would have to wait until you were established in the approach (i.e. past the IAF outbound on the procedure turn).

Avimentor said...

You're not guaranteed obstruction clearance until you're established on a segment of the approach, so you don't descend until you're established. Hence the phraseology in the example I cited: "Maintain 3000 until established ..."

Aviatrix said...

We're not talking about starting descent to MDA, or descending before established on the localizer. WE're talking about being en route at 17,000' with an MEA of 15,000. In Canada as soon as we're cleared for the approach, we can go down to the MEA, then as we get closer, to the 100 nm safe altitude on the lpate, then the 25 nm sector altitude, then any altitude that applies, i.e. passing a radial on a DME arc, with no further descent clearance. In my Canadian IFR course I was warned that in the US I should not leave the en route altitude for the MEA until given a specific descent clearance, separate from the approach clearance. Is this wrong?

Avimentor said...

Okay, that's a much more specific question.

In the US, you maintain your last assigned altitude until you receive the approach clearance. ATC usually will not give you an approach clearance unless you are established on some segment of the approach or they will give you an altitude to maintain until established.

If you are flying to an initial approach fix and will be doing a procedure turn, they will give you an altitude to maintain until you cross that fix.

If you are assigned a STAR (Standard Terminal Arival Route), you maintain your last assigned altitude unless the controller says "Barnburner 123, DESCEND via the ZIGGY 3 arrival."

Aviatrix said...

Sounds like "cleared for the approach" means "cleared to descend to any safe and legal altitude that suits you" on both sides of the border.

In Canada you might be cleared for the approach a long way back and it's possible that you would lose radio contact with the IFR controller before you were established on anything. You'd just be talking to aerodrome traffic or maybe a flight service specialist on the field. It's up to the pilot to step down appropriately.

At a Canadian airport with radar service and an ILS, you would typically be assigned altitudes all the way to the localizer, but if you're given a vector to intercept the localizer cleared for the approach no one will be bothered if you decide you want to go down to the sector altitude depicted on the plate instead of holding that assigned.

Aviatrix said...

The above should be "if you're given a vector to intercept the localizer AND cleared for the approach."

Left out the AND.

It's a pain we can't edit comments.

Anonymous said...

LPV stands for Lateral Precision with Vertical guidance not Localizer Precision.

Anonymous said...

Does anyone know what "APV" means? It is found in the CAP GEN (Jan 18,2007) page 13, Operatng Minima Approach

Aviatrix said...

If you'd left your e-mail I'd tell you, but maybe you'll be back to check. “APV” means approach procedure with vertical guidance; (APV)
(amended 2006/12/01; no previous version)
. See the interpretation section of the CARs or presumably the equivalent section of the latest CAP GEN.

Anonymous said...

Since NP approaches are not always lined up with the runway, when can you legally deviate from a non-precision approach to get into a position to land? Is it as soon as you have the field in sight or at some other point?

Anonymous said...

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Thank you for ANY suport!


Anonymous said...

It's probably much too late to help cj, but maybe someone else finding this will want an answer too:

"Since NP approaches are not always lined up with the runway, when can you legally deviate from a non-precision approach to get into a position to land? Is it as soon as you have the field in sight or at some other point?"

(Disclaimer: these are US procedures; someone should speak up if Canadians do it differently. We already know they do it stylishly!)

In the US anyway, NP approaches that aren't more-or-less aligned with the runway don't have straight-in landing minima, only what are called circling minima.

In any circle-to-land approach, you fly a standard approach until you either break out, or execute a missed approach.

If you break out, then once you're close enough (see below), you maneuver to align yourself and land on the runway of your choice. This is the situation on an unaligned approach.

As for your real question -- when can you maneuver? -- it depends on your approach category. Approach Category is based on how slow the airplane can safely fly.

If you fly a slow Category A airplane like I do, you must do all of your maneuvering within 1.3 miles of a runway. If you fly a Category D 747, you get 2.3 miles to play with.

All that is to say that you must stay on the published approach until 1.3 - 2.3 miles of the airport, depending on Approach Category.

See also this diagram from the FAA Aeronautical Information Manual.

Anonymous said...

Just fyi, the ASA instrument oral exam guide states that LPV stands for Localizer Performance with Vertical Guidance. Not lateral precision. P4-18

Boojangles said...

"LPV" actually stands for "Localizer Performance and Vertical Guidance" xD lol... don't believe me? Ask my boss(DER). Oh, DER is a Designated Engineering Representative. That means they work on behalf of the FAA.

Boojangles said...

Dangit, sorry about the repeat.