IFR 12: DME Arcs
I’ve decided that an CFII (an instrument flight instructor) is a person who never gets to fly, but who knows local holding patterns by recognizing the farms below.
Today is the real lesson 12 and we’re doing a localizer back course, intercepting a VOR radial, a DME arc, a hold, and a circling approach. The previous lesson 12 was an opportunity we had to take for the weather and is credited to tasks in future lessons. An instructor at my school made up this routine. It’s a good practice route for a number of things.
The winds are pretty calm at altitude today and we should have a smooth ride. I’m flying with instructor M today and he takes off from our home airport while I’m wearing foggles. He gives me the controls 200 feel AGL and I track the localizer outbound.
I only have the one VOR receiver that I’m using for the localizer. So we’re using the GPS for the distance to find the fix to use to turn to the VOR beacon. That comes up soon and I turn to the VOR and track that. We overshoot the DME distance we planned while discussing a point, so M gives me a new shorter distance and I track that.
There are two methods to track a DME arc. The one that’s generally taught is to follow a series of short straight approximations to the curve and adjust the VOR and heading every ten degrees. This requires a lot of turning and twisting of the airplane and of various knobs. But this has the advantage of always giving you a heading to fly and is the method we’re practicing today. Instructors M and R are tolerant of my oddball ideas but teach the straight methods.
The second is from (once again) Joe Campbell’s CFII Charles Harris. [http://www.campbells.org/Airplanes/Diary/day7.html] In this method you simply turn closer or further turn to maintain your correct distance from the VOR. (There some other details about entering to make sure you are joining the arc in the right direction too.)
We track that around the arc and I’m late on wind correction as we swing around the VOR. I slide out of the 7 mile arc past 8 miles. I tell M, “I just busted my checkride, I’m over a mile off.” I start working to get back in, but I’m also turning in as well. So it takes a pretty big turn to get re-established. By then I’m almost on my outbound radial so start turning outbound.
We continue on the outbound radial to my inbound side of my hold (also inbound to the aiport). This is a direct entry and so I just turn into the hold. I’m late starting my turn though and have to compensate on the opposite leg. But aside from that and adjusting for wind correction, the hold seems to go well.
We do this several times. M asks me how I’d be prepared for the hold, I comment about slowing down, possibly adding 10 degrees of flaps, checking weather, etc. But I’m not really sure what he’s asking. He then asks about continuing from the hold to landing (a circling approach) and I give similar answers, and discuss the descent gradient. Again, it’s seems all straightforward I think, I’m not sure I’m answering what he’s asking. We’re just orbiting the hold. It is all going much more smoothly than a few lessons ago. The smooth air helps, but the sim practice at home helps too.
In one orbit around the hold I level outbound on the wrong heading for a moment. I catch it quickly and correct. M’s looking out the window and says, “Yes, that’s a better heading.” He knows this hold pattern by watching the ground.
He takes the plane and gives me a new entry to the hold - a teardrop. I do that and go around the hold again. Then M says to approach the airport as I would normally after the hold. On the inbound leg of the hold I slow the plane and put down 10 degrees of flaps to help descend. I come down to 3000 ft, the minimum altitude for this hold.
After I cross the fix I start to descend in earnest. The feet per mile descent is easily figurable from the approach plate. I’m trying to do the math for the feet per minute descent rate I need though, my neurons are too busy. I’m not getting it. So I guesstimate it. M comments, “On a non-precision approach you really need to get down fast. I want to see at least 600-700 fpm decent rate.” Clearly, my guesstimate isn’t very good, something else to work on.
I break out of my simulated clouds at 1300 feet and take off the foggles for the first time. We do the circling approach by crossing the airport then turning downwind. This sort of approach is low, only 500 feet over the airport instead of the pattern altitude of twice that. I’m not used to this sight picture on downwind. I have to compensate for the lower altitude and start my descent later, but it all works out.
Company traffic is also in the pattern. I recognize the voices later, but I’m busy enough not to recognize them now.
We do a touch-and-go, then depart back toward home. M tells me to just take us home, no need to use the foggles. We’ve accomplished the lesson. He never gets to fly, so as long as this isn’t instructional, I offer the controls to him. He flies us back to our home airport pointing out landmarks along the way. It’s novel to be able to see outside the plane again! More company traffic is following us in the pattern, I do the landing when we’re back. It’s not my best landing, I’m picky about that. I should probably just go practice a few landings sometime soon but the annual is next week.
April 1st, 2009 at 13:46
For your overhead, you circle down to 500′ agl, cross the field then turn downwind but you are now 500′ beneath the “regular” traffic? I have not seen that.
Here in Canada we do not have the “standard 45″ entry. For us a standard entry would involve crossing the field from the “upwind” side at TPA (1000′ AGL) over the center of the runway then making a 90 degree turn to downwind. You would then perform a landing beginning from the abeam position. This works well as you are at the same alt as any other downwind traffic and it makes it eaasy to spot them. (and during your turn in makes it easy for them to spot you). I’m curious how this seperation is made if you have a 500′ altitude difference.
April 1st, 2009 at 16:46
I think the idea is that if you’re doing an IFR approach to a pilot-controlled field, then no one is flying VFR. In IFR it’s called a circling approach. Clearly this VFR/IFR seperation breaks down sometimes. In practice what I’ve seen is that incoming IFR pilots will break off their approach and join the downwind (or whatever) for landing in the pattern like VFR pilots are doing. This is one of the reasons that circling approaches are riskier than other approach types.
If we’d had traffic for that flight, then that’s what we would have done too. But we were alone at that field right then. I’m doing a lot of my IFR training at night - that helps for traffic conflicts.
Your Canadian overhead entry is used in the US sometimes, but isn’t commonly liked. As you say the US FAA encourages the 45 degree entry.