Some Thoughts about Instrument Approaches in Light Aircraft
5 years 2 months ago #15600
by Westcoast
<p class="MsoNormal"><img src="coppermine/displayimage.php?pid=1429#top_display_media" alt="">I’ve been flying Larry Chew’s new Alaska Marine Highway Tour the last week or so. By in large these flights are what one my old bosses (an ex radio ham) would have called “arm chair copy”, meaning that no real effort is required. I’ve been flying my Aerosoft DHC-6 Twin Otter (extended version). It’s a simple aircraft, with no complicated flight computer, and it’s quite capable, with twin turboprops, reversible props, tricycle gear and tundra tires. It’s equipped with a GPS unit, so flying these point-to-point flight plans between small airports in the Alaskan panhandle is plug simple. Just take off, climb to a safe enroute altitude, slave the autopilot to the GPS and sit back and enjoy the view. Presently, and it doesn’t take more than 30 or 40 minutes, the destination strip appears in the distance and you’re ready to descend and land, bada-bing. In spite of their simplicity, I enjoy these easy, relaxing flights, kind of like playing solitaire and easy enough that I can do something else while I fly.</p>
<p class="MsoNormal">I customarily fly in today’s real weather, courtesy of Active Sky Next, and I’ve been pleasantly surprised that the Alaskan weather has been relatively stable and clear for most of the past week. So, I haven’t even bothered to file an IFR flight plan, or use the services of ATC. Nonetheless, for those destination airports with an instrument approach procedure (IAP), I usually elect to fly that, both because it makes the flight more interesting and because it helps to keep my IFR skills current. Real World IFR pilots have to do a lot of flying, either under the hood or in instrument meteorological conditions (IMC), to maintain a current IFR ticket. I’m not that disciplined, though I probably should be, I just look out the window as I fly the procedure. However, given that the Twotter has no flight computer or integrated navigational database, I’m stuck with relying on the GPS unit to identify the waypoints of the IAP. This is a little tricky, as the nav database in the GPS unit is not current and so the procedure waypoints don’t correspond to those on the current FAA instrument procedures charts. That is, I am not working with the current Aeronautical Information and Control (AIRAC) database. I suppose that there might be some way to update my GPS database, but I’m too lazy to spend the time required to look into that. As a result, there is always that awkward moment when I have to switch over from hands-off GPS navigation to more active radio navigation.</p>
<p class="MsoNormal">Most often the IAP I use will be a straightforward ILS approach. These are really easy to fly, and with a good autopilot, they make a good landing easy to do. In a B738 I can land like a champ, without my passengers even noticing that we’ve touched down. In the Twotter it’s not quite so easy. First of all, the AP has more trouble tracking the localizer and glideslope, then the high wings mean the ship is a good deal more sensitive to cross winds than the big iron and so it’s hard to hold the centerline on final. Finally, there’s no auto throttle and the PW turboprops are very slow to spool up or down. So, I’m often find myself chasing Vref all the
way down the glide slope, and end up spooling up just above the threshold and then having to set her down hard before I run out of runway. Not as pretty as I would like, and I then am duly outed and chastised by the Econ tracking software I helped to build. Oh, cruel fate. On top of that, I’ll confess that I sometimes fail to continue a far side approach far enough to get under the glide slope (impatience) or forget (memory problems) to switch the auto pilot’s Nav data source from the GPS receiver to the LOC receiver before beginning the approach and then discover my oversight too late to make a clean approach. I suppose this may be why they don’t let octogenarians (I’ll be there next year) fly commercial aircraft.</p>
<p class="MsoNormal">Yesterday, I flew flight 900009 from Wrangell, Alaska (PAWG) to Kake, Alaska (PAKE), a short hop of 58 NM, which I could have completed in about a half-hour with a near side or short final approach to PAKE. PAKE was reporting 100/06, 10 SM and 3700 BKN, so there was no reason to fly an instrument approach. But, consistent with my pattern, I selected an IAP. Two were available, an RNAV approach or an NDB/DME approach, both to runway 11. The Twotter is not equipped for RNAV approaches, but it does have both an ADF and ILS/DME receivers, so I opted for the NDB approach. </p>
<img src="coppermine/albums/userpics/10009/22/normal_09231ND11_0001.png" alt="NDB Approach to Rwy 11 at PAFE" width="658" height="1010">
The non-directional beacon (NDB) is the oldest and most primitive of the radio navigational aids still in use. It simply tells the automatic direction finder (ADF) receiver in the aircraft the direction to the beacon; that is the ADF needle shows the direction to the NDB transmitter, relative to the current heading of the aircraft. You might think this is enough for rough radio navigation, and it is. But it isn’t enough for an instrument approach. The beacon will lead you to the transmitter, which may/may not be collocated with the airstrip, but even if it is, your approach can be from any direction, and will not, generally, be aligned with the runway. That may still be adequate if the ceiling is high enough to permit a “circle to land” approach and if high terrain does not make such an approach impossible. However, in general an NDB approach requires that you track a given inbound radial to the beacon, which will generally align you (more-or-less) with the runway. At least, it will keep you clear of the terrain on your trip down the glide path. Of course, NDB approaches also require that you descend along a tightly defined glide path (without the provision of a glide slope signal). This is what makes the collocated distance measuring equipment (DME) transmitter so useful, it tells you how far you are from the beacon, and thus how high you should be. In the case of the NDB/DME Rwy 11 approach at PAKE, the approach course is 085 deg and is not aligned with the runway at 111 deg. However, the missed approach point at 1100’ (934’ AGL) is far enough from the runway threshold to allow the pilot to bank into the runway heading before touching down. If the runway isn’t visible from that point, you go missed and return to Wrangell, in the company of a grumbling plane load of unhappy passengers.
<p class="MsoNormal">It’s well-know among pilots that NDB approaches are hard to fly. In fact, I was watching a utube video of a Lufthansa A318 approach to (I think) Munich just this afternoon. When the approach controller switched the approach from an ILS approach from the east to an NDB approach from the west, the copilot remarked, “oh, those are always fun”. I have a friend who is a retired Marine and commercial helicopter pilot. We were discussing NDB approaches yesterday an he observed that he's not sure he could fly one today if he had to. So, what's so hard about these approaches? Let me try to explain in the following paragraph.</p>
<p class="MsoNormal">So, you just have to track the inbound 085 radial to the NDB (outbound 265 radial); how hard can that be? If your aircraft heading is 085 deg and the ADF needle is pointing straight ahead, you’re on the 85 deg. radial. Suppose your aircraft heading is 090 deg. And the ADF points 030 deg to the left? What inbound radial are you on? To get that answer, you turn the dial on your “fixed card” ADF indicator until the heading at the top of the dial is the same as the aircraft heading, and read the radial off the card: 060 deg. That's pretty simple, right? The ADF card is just doing the subtraction (90 - 30 = 60) for you. You could do that in your head. So, at that instant your aircraft is positioned on the 60 deg inbound radial, but not traveling inbound along that radial. Instead, you are on a heading of 90 deg. heading toward a position that would soon put you on the 50 deg. radial, etc. So, how should you get onto the inbound 085 radial? Clearly, you need to turn to the left. If you were to quickly turn 30 deg. to the left, you would still be on the 60 deg. radial, but now with a heading of 60 deg., so the ADF needle would be pointing straight ahead and you would be inbound on the 60 deg. radial. So clearly you will have to turn even further to the left to get onto the 85 deg. radial. So we could do that by turning a further 30 deg. to the left, tracking the ADF card by hand to align the bearing indication on the top of the ADF with the new heading of the aircraft. Now your new current heading is 30 deg.(90 - 30 - 30 = 30) and the ADF needle is pointing 30 deg. to the right, to 60 deg. You stay on this heading as the ADF pointer needle moves steadily further to the right until it is reading 085 deg., and then you turn right to join that bearing. This is made easier if your aircraft has a radio magnetic indicator (RMI) instead of a fixed card ADF. An RMI includes the aircraft heading indicator and automatically rotates the ADF card to align instantaneously with the aircraft heading, so you don’t have to rotate the card by hand as your aircraft turns. A simple thing, but important when you’re really busy with other tasks. Anyhow, the Twotter has a fixed card ADF, not an RMI, so, as you make course adjustments, you have to keep rotating the ADF. While you were doing this, what happened to your airspeed and altitude? Are you still on the specified glide path? [You’re starting to get the point]. It gets even worse without the DME, because you have to keep track of your distance from the beacon using time and airspeed, and even worse if you have a crosswind, in which case your track is different from your heading. Now you see why it’s said that some professional pilots never learn to fly an NDB approach. But if you were flying SPA flight 900009 in the Twin Otter yesterday afternoon and the ceiling was 1200’, you would either have had to fly the NDB approach or take your passengers back to Wrangell.</p>
<p class="MsoNormal">Anyhow, I flew the NDB approach yesterday and it wasn’t pretty. I touched down OK, but it isn’t clear that the instrument approach made my approach better than it would have been if I had just flown the visual; probably not. If I had been under the hood, I probably would have had to go around, probably more than once.</p>
<p class="MsoNormal">So, what might I (we) do about this? I’m so glad you asked. Mosey over to the right-hand menu on the SPA home page. See that heading labeled “Training Department”? If you open that, you will see a number of training flights. Maybe you flew an few of those when you first joined SPA (probably not). In particular, there’s one labeled TP 8905 - NDB Approach to Great Bend (KGBD). In that training flight our esteemed former Training Manager, Jerry Allen (aka “Jet Jerry”) explains how to fly an NDB approach for real. Jerry is a retired Air Transport Pilot (ATP) and has many, many hours of RW flying experience. He started as an Army Caribou pilot in Viet Nam and ended his career flying the MD 80 series aircraft for several large commercial carriers. I have learned a great deal from Jerry and he is continuing to instruct me. I’m going to go fly TP 8905 until I get this NDB procedure right. I suggest you do the same. While you’re at it, try the DME Arc approach training flight too. That’s the approach that an Air Force pilot screwed up going into Dubrovnik, Croatia in 1996, killing Commerce Secretary Ron Brown and 32 American business executives. These things are harder than they look, particularly in the dark in bad weather.</p>
<p class="MsoNormal">Cheers,</p>
<p class="MsoNormal">Mike
P.S. In the (probably unlikely) event that any of you has read down to this point and been tempted to try TN 8905, let me warn you that the Hilyn (HL 338 kHz) approach featured in this training flight is no longer available on current charts. There is a NDB approach to KGBD, but it is for Rwy 35, instead of Rwy 11 and uses GB (419 kHz). However, my Plan G database, derived from my FSX, still shows HL and so, presumably, the older approach could still be flown in FSX, assuming that I could read the charts in Jerry's training flight, as they are no longer available from current on-line sources.</p>
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<p class="MsoNormal"> </p><br /><br /><!-- editby --><br /><br /><em>edited by: Westcoast, Dec 04, 2019 - 01:34 PM</em><!-- end editby -->
<p class="MsoNormal">I customarily fly in today’s real weather, courtesy of Active Sky Next, and I’ve been pleasantly surprised that the Alaskan weather has been relatively stable and clear for most of the past week. So, I haven’t even bothered to file an IFR flight plan, or use the services of ATC. Nonetheless, for those destination airports with an instrument approach procedure (IAP), I usually elect to fly that, both because it makes the flight more interesting and because it helps to keep my IFR skills current. Real World IFR pilots have to do a lot of flying, either under the hood or in instrument meteorological conditions (IMC), to maintain a current IFR ticket. I’m not that disciplined, though I probably should be, I just look out the window as I fly the procedure. However, given that the Twotter has no flight computer or integrated navigational database, I’m stuck with relying on the GPS unit to identify the waypoints of the IAP. This is a little tricky, as the nav database in the GPS unit is not current and so the procedure waypoints don’t correspond to those on the current FAA instrument procedures charts. That is, I am not working with the current Aeronautical Information and Control (AIRAC) database. I suppose that there might be some way to update my GPS database, but I’m too lazy to spend the time required to look into that. As a result, there is always that awkward moment when I have to switch over from hands-off GPS navigation to more active radio navigation.</p>
<p class="MsoNormal">Most often the IAP I use will be a straightforward ILS approach. These are really easy to fly, and with a good autopilot, they make a good landing easy to do. In a B738 I can land like a champ, without my passengers even noticing that we’ve touched down. In the Twotter it’s not quite so easy. First of all, the AP has more trouble tracking the localizer and glideslope, then the high wings mean the ship is a good deal more sensitive to cross winds than the big iron and so it’s hard to hold the centerline on final. Finally, there’s no auto throttle and the PW turboprops are very slow to spool up or down. So, I’m often find myself chasing Vref all the
way down the glide slope, and end up spooling up just above the threshold and then having to set her down hard before I run out of runway. Not as pretty as I would like, and I then am duly outed and chastised by the Econ tracking software I helped to build. Oh, cruel fate. On top of that, I’ll confess that I sometimes fail to continue a far side approach far enough to get under the glide slope (impatience) or forget (memory problems) to switch the auto pilot’s Nav data source from the GPS receiver to the LOC receiver before beginning the approach and then discover my oversight too late to make a clean approach. I suppose this may be why they don’t let octogenarians (I’ll be there next year) fly commercial aircraft.</p>
<p class="MsoNormal">Yesterday, I flew flight 900009 from Wrangell, Alaska (PAWG) to Kake, Alaska (PAKE), a short hop of 58 NM, which I could have completed in about a half-hour with a near side or short final approach to PAKE. PAKE was reporting 100/06, 10 SM and 3700 BKN, so there was no reason to fly an instrument approach. But, consistent with my pattern, I selected an IAP. Two were available, an RNAV approach or an NDB/DME approach, both to runway 11. The Twotter is not equipped for RNAV approaches, but it does have both an ADF and ILS/DME receivers, so I opted for the NDB approach. </p>
<img src="coppermine/albums/userpics/10009/22/normal_09231ND11_0001.png" alt="NDB Approach to Rwy 11 at PAFE" width="658" height="1010">
The non-directional beacon (NDB) is the oldest and most primitive of the radio navigational aids still in use. It simply tells the automatic direction finder (ADF) receiver in the aircraft the direction to the beacon; that is the ADF needle shows the direction to the NDB transmitter, relative to the current heading of the aircraft. You might think this is enough for rough radio navigation, and it is. But it isn’t enough for an instrument approach. The beacon will lead you to the transmitter, which may/may not be collocated with the airstrip, but even if it is, your approach can be from any direction, and will not, generally, be aligned with the runway. That may still be adequate if the ceiling is high enough to permit a “circle to land” approach and if high terrain does not make such an approach impossible. However, in general an NDB approach requires that you track a given inbound radial to the beacon, which will generally align you (more-or-less) with the runway. At least, it will keep you clear of the terrain on your trip down the glide path. Of course, NDB approaches also require that you descend along a tightly defined glide path (without the provision of a glide slope signal). This is what makes the collocated distance measuring equipment (DME) transmitter so useful, it tells you how far you are from the beacon, and thus how high you should be. In the case of the NDB/DME Rwy 11 approach at PAKE, the approach course is 085 deg and is not aligned with the runway at 111 deg. However, the missed approach point at 1100’ (934’ AGL) is far enough from the runway threshold to allow the pilot to bank into the runway heading before touching down. If the runway isn’t visible from that point, you go missed and return to Wrangell, in the company of a grumbling plane load of unhappy passengers.
<p class="MsoNormal">It’s well-know among pilots that NDB approaches are hard to fly. In fact, I was watching a utube video of a Lufthansa A318 approach to (I think) Munich just this afternoon. When the approach controller switched the approach from an ILS approach from the east to an NDB approach from the west, the copilot remarked, “oh, those are always fun”. I have a friend who is a retired Marine and commercial helicopter pilot. We were discussing NDB approaches yesterday an he observed that he's not sure he could fly one today if he had to. So, what's so hard about these approaches? Let me try to explain in the following paragraph.</p>
<p class="MsoNormal">So, you just have to track the inbound 085 radial to the NDB (outbound 265 radial); how hard can that be? If your aircraft heading is 085 deg and the ADF needle is pointing straight ahead, you’re on the 85 deg. radial. Suppose your aircraft heading is 090 deg. And the ADF points 030 deg to the left? What inbound radial are you on? To get that answer, you turn the dial on your “fixed card” ADF indicator until the heading at the top of the dial is the same as the aircraft heading, and read the radial off the card: 060 deg. That's pretty simple, right? The ADF card is just doing the subtraction (90 - 30 = 60) for you. You could do that in your head. So, at that instant your aircraft is positioned on the 60 deg inbound radial, but not traveling inbound along that radial. Instead, you are on a heading of 90 deg. heading toward a position that would soon put you on the 50 deg. radial, etc. So, how should you get onto the inbound 085 radial? Clearly, you need to turn to the left. If you were to quickly turn 30 deg. to the left, you would still be on the 60 deg. radial, but now with a heading of 60 deg., so the ADF needle would be pointing straight ahead and you would be inbound on the 60 deg. radial. So clearly you will have to turn even further to the left to get onto the 85 deg. radial. So we could do that by turning a further 30 deg. to the left, tracking the ADF card by hand to align the bearing indication on the top of the ADF with the new heading of the aircraft. Now your new current heading is 30 deg.(90 - 30 - 30 = 30) and the ADF needle is pointing 30 deg. to the right, to 60 deg. You stay on this heading as the ADF pointer needle moves steadily further to the right until it is reading 085 deg., and then you turn right to join that bearing. This is made easier if your aircraft has a radio magnetic indicator (RMI) instead of a fixed card ADF. An RMI includes the aircraft heading indicator and automatically rotates the ADF card to align instantaneously with the aircraft heading, so you don’t have to rotate the card by hand as your aircraft turns. A simple thing, but important when you’re really busy with other tasks. Anyhow, the Twotter has a fixed card ADF, not an RMI, so, as you make course adjustments, you have to keep rotating the ADF. While you were doing this, what happened to your airspeed and altitude? Are you still on the specified glide path? [You’re starting to get the point]. It gets even worse without the DME, because you have to keep track of your distance from the beacon using time and airspeed, and even worse if you have a crosswind, in which case your track is different from your heading. Now you see why it’s said that some professional pilots never learn to fly an NDB approach. But if you were flying SPA flight 900009 in the Twin Otter yesterday afternoon and the ceiling was 1200’, you would either have had to fly the NDB approach or take your passengers back to Wrangell.</p>
<p class="MsoNormal">Anyhow, I flew the NDB approach yesterday and it wasn’t pretty. I touched down OK, but it isn’t clear that the instrument approach made my approach better than it would have been if I had just flown the visual; probably not. If I had been under the hood, I probably would have had to go around, probably more than once.</p>
<p class="MsoNormal">So, what might I (we) do about this? I’m so glad you asked. Mosey over to the right-hand menu on the SPA home page. See that heading labeled “Training Department”? If you open that, you will see a number of training flights. Maybe you flew an few of those when you first joined SPA (probably not). In particular, there’s one labeled TP 8905 - NDB Approach to Great Bend (KGBD). In that training flight our esteemed former Training Manager, Jerry Allen (aka “Jet Jerry”) explains how to fly an NDB approach for real. Jerry is a retired Air Transport Pilot (ATP) and has many, many hours of RW flying experience. He started as an Army Caribou pilot in Viet Nam and ended his career flying the MD 80 series aircraft for several large commercial carriers. I have learned a great deal from Jerry and he is continuing to instruct me. I’m going to go fly TP 8905 until I get this NDB procedure right. I suggest you do the same. While you’re at it, try the DME Arc approach training flight too. That’s the approach that an Air Force pilot screwed up going into Dubrovnik, Croatia in 1996, killing Commerce Secretary Ron Brown and 32 American business executives. These things are harder than they look, particularly in the dark in bad weather.</p>
<p class="MsoNormal">Cheers,</p>
<p class="MsoNormal">Mike
P.S. In the (probably unlikely) event that any of you has read down to this point and been tempted to try TN 8905, let me warn you that the Hilyn (HL 338 kHz) approach featured in this training flight is no longer available on current charts. There is a NDB approach to KGBD, but it is for Rwy 35, instead of Rwy 11 and uses GB (419 kHz). However, my Plan G database, derived from my FSX, still shows HL and so, presumably, the older approach could still be flown in FSX, assuming that I could read the charts in Jerry's training flight, as they are no longer available from current on-line sources.</p>
<a href="coppermine/displayimage.php?pid=1429#top_display_media"><img src="coppermine/displayimage.php?pid=1429#top_display_media" alt="" width="400" height="520"></a>
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<p class="MsoNormal"> </p><br /><br /><!-- editby --><br /><br /><em>edited by: Westcoast, Dec 04, 2019 - 01:34 PM</em><!-- end editby -->
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