Sunday, March 22, 2009

Hold Everything, Part III

For being so simple, holding patterns are a surprisingly deep topic. In this installment I'll cover some subtle points like orientation on the outbound leg, when to start timing, the 5 Ts, Dealer's Choice Entry, wind correction, station side holds, and the so-called Crisis Entry.

You Gotta Believe
When a photojournalist is mired in a crowd of people and is unable to see the subject they are trying to photograph, they often hold their camera up, point it in the generally correct direction, and click the shutter in hopes of getting a usable shot. This used to be called a Hail Mary, a term also applied to a quarterback throwing a football a great distance downfield in hopes that his receiver will be there to catch it.

An aviation equivalent of a Hail Mary is something that many new instrument pilots have a hard time grasping: Without a GPS depiction of a holding pattern you really have no positive course guidance on the outbound leg. On the inbound leg, the CDI or HSI will be centered and the ADF needle should basically be pointing straight up (aside from any wind correction angle).

If you're navigating with a VOR or ADF, you cross the fix, turn outbound, fly for a minute and see what happens when you turn inbound. If you have an idea of the winds aloft prior to entering the hold, that knowledge can certainly inform your choice of heading; more on that later.

With a GPS depiction of the holding pattern, you'll have an excellent idea of where the desired outbound leg is. Some GPS receivers capable of depicting holding patterns will even change the shape of the holding pattern race track to account for your aircraft's speed and for wind correction.

Here's the G1000's predicted holding pattern shape at 145 knots with winds out of the NNE at 25 knots.

Here's the same predicted holding pattern shape after slowing to 105 knots.

Without GPS, the first time around the hold is often a bit of a gimme - You get a better idea of the winds aloft and refine the hold with each lap you fly. This can be a good thing if you have to hold for a long time because it gives you something to do and relieves the boredom.

Five is Enough
A popular checklist mnemonic used as you cross a holding fix (or any other fix, for that matter) is the Five Ts. Some instructors teach a slightly different order and some even teach more than Five Ts. Here's my version:
  • Turn - to the outbound heading
  • Time - start your time, if necessary
  • Twist - twist the course pointer or OBS to the INBOUND course, if necessary
  • Throttle - reduce to holding speed if you haven't already done so
  • Talk - report entering the hold
Some folks insist that you start timing before you start turning, but it doesn't really matter as long as you are consistent in which you choose to do first. I teach turn before time because it works well when flying one of those rare instrument approaches that have a course change over the final approach fix.

I've seen pilots and instructors who insist on twisting the OBS or course pointer when turning outbound as well as when turning inbound in the hold. That seems a bit kooky in light of the above discussion of course guidance on the outbound leg, but I guess it's harmless as long as you keep your situational awareness.

I rarely see pilots report entering the hold, even though it's one of the compulsory reports under IFR. The format goes something like this:
Barnburner 123, entered the hold, Sacramento, 2132 Zulu.

Get Your Fix
An often misunderstood concept is when to start timing the outbound leg of the hold. When you are entering a hold using a teardrop or parallel entry, start the outbound timing when you cross the fix. When flying a direct entry or once established in the hold, start timing abeam the holding fix provided you can determine that position.

Station passage over a VOR occurs on a teardrop or parallel entry as soon as your VOR receiver shows a positive reversal of the To/From flag. For a hold over an NDB, your ADF needle will start to point behind your wing when you've passed the station. If you are holding over a VOR intersection or a DME fix on a VOR radial, the equivalent of station passage is when you have passed the cross-radial or DME distance that defines the holding fix.

If you are navigating using GPS, you need to suspend waypoint sequencing (by pressing the OBS button) unless you are holding over a missed approach holding waypoint defined in an instrument approach procedure. The nice thing here is that when waypoint sequencing is disabled and you cross the fix, the To/From flag will reverse just like when crossing a VOR station.

If you did a direct entry or are already established in the holding pattern, start timing the outbound leg when abeam the holding fix. Holding over an NDB, abeam the station occurs when the ADF needle is basically pointing off the wing. Holding over a VOR and assuming you set the proper inbound course on your OBS or HSI, abeam the station occurs when the To/From flag flips to TO.

Holding over a GPS waypoint with waypoint sequencing disabled and the proper inbound course set, abeam the station occurs when the To/From flag flips to TO.

When holding over the intersection of two VOR radials, you won't have a good indication of the position abeam the fix unless you are lucky enough to have a cross-radial is perpendicular to the inbound course. In these cases, start timing as you cross the fix and turn outbound, and after a minute you should have completed your 180 degree turn and you can start timing the outbound leg.

Holding over a DME fix on a radial without DME leg lengths, start timing when the DME distance is the same distance as that which defines the holding fix.

Dealer's Choice
When you are approaching the holding fix and your heading is within ±10˚ of the outbound course, it's really up to you as to whether you do a teardrop entry or a parallel entry. One advantage of doing a parallel entry is when winds aloft are strong. With a strong wind perpendicular to the inbound and outbound courses, tracking outbound on the inbound course provides you with positive course guidance. This means you can determine a wind correction angle for the outbound leg and apply that knowledge to flying the inbound leg.

Here's the missed approach holding for the Stockton ILS RWY 29R. Note that the G1000 is defaulting to a teardrop entry and it's probably best to follow that advice. Without GPS and using just plain old VORs, you could do a parallel or teardrop - your choice.

Wind Correction
When there's a strong wind perpendicular to the inbound and outbound legs, your holding pattern will end up looking egg-shaped rather than the idealized, symmetrical racetrack pattern. In some cases the wind will increase your groundspeed on the outbound leg and decrease your groundspeed on the inbound leg. Unless your holding instructions specified DME leg lengths, you'll need to apply a little trial and error to adjust your outbound timing so that your inbound leg takes a minute.

One rule of thumb is that whatever wind correction angle you needed on the inbound leg should be doubled or tripled in the opposite direction for the outbound leg.

Near Perpendicular Entry
You may find cases where a direct entry has you flying course almost perpendicular to the outbound course.

If you cross the fix and turn outbound, your outbound course will be so close to the inbound course that you're guaranteed to overshoot the turn to the inbound course. In these cases it can be beneficial to cross the fix and wait 10 or 15 seconds before turning outbound to but some distance between your outbound course and the inbound course. In fact, that is what the G1000 is programmed to do in these situations.

Station-side Holds
Remember that step for determining the outbound heading where you ask if the cardinal direction matches the radial? Most of the time they will match, but when they differ is called the station-side hold. This occurs where the holding fix is defined as a DME distance on a VOR radial and the holding pattern is located on the same side of the fix as the VOR station.

Look at this illustration and assuming North is up, it's pretty easy to see that the cardinal direction for this hold would be East, but the radial specified would be 270 degrees. This is where you determine the reciprocal of 270 is 090, write it down as your outbound heading, and turn to that heading after crossing the fix. To navigate on the inbound leg, your CDI or HSI should be set to 270.

Crisis Entry
Try as you might, you may get flummoxed, not be able to determine the entry procedure, and screw up your first trip through the hold. The best thing to do turn back to the holding fix (left or right turn, whichever is shortest), figure out the outbound heading, cross the fix and turn outbound again. You don't want this to happen on a check ride, but everyone makes mistakes from time to time. In real life, think of it as a mulligan or a do-over.

This concludes my series on holding. I hope you've found it helpful and enlightening!


Ron said...

Nice series! I will refer students to this when training them on the glass panel aircraft. One problem with the G1000 is that there are so many software versions out there that it's sometimes difficult to remember what one version does that another doesn't do. Where I work, we have three airplanes with the G1000 system: two DA40s and an SR22 G3 turbo with the Perspective avionics suite. That one obviously has the synthetic vision, integrated autopilot, keyboard, etc. But the two DA40s are identical except one has a newer version of the software, so the engine gauges, menus, and other stuff is a bit different. This is one of the problems I see emerging with the G1000: so many software versions out there that even if you're proficient with that panel, you might get some surprises. Not what you need when flying hard IFR.

Matthew said...

Nice post. Holding is something that happens very frequently in checkrides and very rarely in real life (at least here in the UK and western Europe). My passengers couldn't understand why I got so excited when I was asked to hold at an arbitrary waypoint by Paris Control last year. It was my first actual hold as an instrument-rated pilot in three years! Turns out that everyone in the control tower at my destination had gone for lunch and the en-route controller couldn't get hold of them to hand me over. In the end he gave up and had me call him when I landed safely so he could close my flight plan. This was an biggish, IFR-capable regional airport, not some grass strip. But in France, lunch is sacrosanct! :)