IFR REFRESHER
MICHAEL CHURCH
MAY, 2001 

MORE T'S

With the great variety of items competing for your attention on the average instrument approach, it is useful to have mental checklists handy to trigger necessary actions at the appropriate times. Although workload is demanding all through the approach, two locations, the final approach fix on non-precision approaches and the outer marker on the precision variety, tend to require the highest number of actions in the shortest time.

Last month, I started in on the T's–the ubiquitous mental checklist used by most IFR pilots at those two points. In the version in use in my cockpit, there are six items, and the order is "Time, turn, throttle, twist, tune, talk." As expected, I’ve already heard from pilots who disagree on both content and sequence. No matter, the similarities far outweigh the differences. The single most important point, of course, is the fact that so many recognize and use some variant of the list at all.

With "Time" and "Turn" out of the way at the conclusion of last month’s column, we are left, again using my personal arrangement, with another four to go: Throttle, Twist, Tune and Talk.

THROTTLE
Any instrument approach can be successfully summarized in this single sentence: a non-visual descent made while tracking a nav aid. The value of this simplified description lies in its emphasis on descent as a primary importance–no matter how beautiful the navigation, no IFR approach ever succeeded without an organized let down. This observation provides the argumnet for placing "throttle" early in the list of T’s.

Non-Precision Approaches: In the T's, done at the final approach fix (FAF) on non-precision approaches, "throttle" serves in general as a trigger to start the final portion of the descent–that is, to reduce power and start losing altitude toward the airport. The actions triggered will also include landing gear extension if that step has not already been taken. The power setting of course depends on the airplane, but it is worth pointing out as a generality that it is desirable to use higher rates of descent on non-precision approaches than on their cousins, the precision variety.

The need for a relatively rapid descent on non-precision approaches results from two circumstances, the first at the beginning, the second at the end of the approach. At the start, the minimum altitude at the FAF is usually higher than at comparable locations on precision approaches. At the other end, there is a need to reach minimum descent altitude (MDA) early: on most non-precision approaches, it is necessary to transition to visual conditions at least three quarters of a mile–in some cases even further–from the threshold if you are to successfully complete the approach. Acting together, these two altitude dynamics should make it clear that a leisurely descent rate on a non-precision approach is likely to create problems: if you are slow getting down to MDA, you risk breaking out from the clouds too late to do anything except the missed approach procedure.

As a consequence, on an approach done with a groundspeed of 120 knots, it is best to use a descent rate–and a power stetting to match–of nearly 1000 fpm. Although this may seem a bit steep, it will pay off when ceilings are close to minimums.

Precision approaches: As pointed out last month, some of the T's are superfluous on precision approaches–others subtly changed in character. "Throttle" is one that changes: by the time you reach the outer marker (OM) on an ILS, it is expected you will be established on the glide slope (GS), making a deliberate power reduction at that point unnecessary. On the other hand, there is a good chance you will not have been established on the GS long enough to have completely settled in, and the "throttle" checklist item can serve as a reminder to check that airspeed is falling into the desired range. With an unexpected tailwind, airspeed will trend high on the glide slope, with a headwind, the reverse—-in either case, the resulting airspeed variation will make a power change from the norm necessary; "throttle" can serve as a handy reminder.

Holding: Again as mentioned last month, the T's are not limited in usefulness to approaches. Upon arrival at a holding fix, "throttle" can serve as a trigger to slow to a more sedate speed than cruise for your sojourn in the holding pattern.

TWIST
We have reached the fourth item of the T’s: "twist," designed to remind you to make a change to the OBS on one of your VOR receivers as you progress through the checklist.

Non-precision approaches: On non-precision approaches, there are two scenarios that require an OBS adjustment. The first involves the existence of a "step down" fix somewhere between the FAF and the airport. Such a fix is placed to create an interim level-off inside the FAF, effectively delaying descent to MDA until the fix has been passed. This setup naturally makes it necessary to pre-set on board nav equipment–usually the #2 VOR–sometime after FAF passage for identification of the "inside" fix. Although this setting can be made anytime prior to actual arrival at the fix, it is best to get it over with early to avoid forgetting it entirely in the press of other chores that inevitably multiply as you approach the airport.

The second circumstance that requires a twist of the OBS on non-precision approaches involves the #1 VOR and a "dogleg" at the FAF–a course change necessary to realign direction toward the airport on the final approach segment. Although the turn to the new course will have already been made as the second item in the T’s, it is now time to adjust navigation equipment to ensure course guidance.

This is one of the major areas where some lists differ from mine, their proponents arguing that the twist is naturally linked to and should therefore directly follow–or perhaps even precede–"turn" on the list: this would have the effect of placing "twist" before "throttle." These are logical arguments, but for reasons already given I feel the importance of starting the descent slightly outweighs that of re-setting the nav equipment, so continue to resist.

There is, however, one circumstance where the order I use is out of sequence–where throttle reduction has to wait on the "twist." It occurs on dogleg approaches when the VOR needle pegs off to one side or the other after the new OBS adjustment has been made, where, in other words, you do not have "live" navigation guidance and will not until you have succeeded in correcting a bit closer to the newly selected course. In that circumstance, safety suggests delaying descent until the needle comes alive on the new course in order to avoid letting down without valid nav guidance.

It can seem a subtle point, because in most cases the off-course indication is merely the result of being close to the cone of confusion directly above the VOR, not an indication of a real navigation error. I have, however, seen the occasional pilot make the wrong OBS selection at the FAF and then start descending on the assumption his off-course indication would self-correct after a bit of forward progress. Of course, with the wrong OBS selection a pilot risks making serious navigation errors with no easy way to recognize the problem–hardly what you want to be doing as you descend within a thousand feet of the terrain. The best advice? With or without a course change at the FAF, don’t just descend automatically after passing the fix–delay "throttle" until you have assured course guidance for your continued progress.

Precision approaches: Both scenarios requiring a "twist" of the OBS during the T’s have no direct relevance to precision approaches. ILS approaches are straight–no turns–and the GS descends all the way to decision height (DH) without level-offs. The only argument for fiddling with an OBS at all during the T's on an ILS is the same one used at the start of the checklist to start timing the approach: the possibility of GS failure and the resulting need to transition to a non-precision localizer procedure.

Holding: On entering holding patterns, there is usually a need to twist the #1 OBS to reflect the course to be flown on the inbound leg. With an intersection hold, this chore may be complicated by the need to switch settings entirely between your two VOR receivers if you didn't navigate to the fix with the VOR you will be using as the primary aid in the hold itself.

TUNE and TALK
Mercifully, the last two T's are simple: tune the appropriate com frequency and talk as directed. On the approach, talking is a simple position report to an interested party: either a tower controller or other pilots using a non-towered airport. In the case of holding pattern entries, the AIM suggests a report of time and altitude to the controlling ATC facility.

That's it for the T's. More important than the details attached to each individual item is the observation I made in the first installment: the checklist does little good unless its items are rehearsed and planned in advance of need. Being able to recite all six (or so) words in machine gun fashion is valueless. But taking the time in advance to consider each item and its possible significance to an upcoming approach can be a real time saver and a genuine aid to consistent and successful performance as you commit to the final descent toward the airport.