I don’t often write about my instructional flights, but I thought this one covered a range of topical IFR approach training issues. An IRR student of mine had moved on to the final section of the syllabus – learning how to fly instrument approaches – and I felt he was good enough to be ready for a more challenging flight. Although the syllabus offers the option of many different types of approach, I tend to focus on the ILS and RNP (sometimes called GPS) since these are by far the most common actually flown today.
VFR pilots may find this a bit technical and perhaps a bit obtuse, but I hope that those with some interest in IFR will find it helpful. It also introduces some recent differences in how GPS may be used, even with relatively older avionics.
My student was flying his own (shared) Cessna 172 equipped with a Garmin 430W GPS navigator box alongside a standard six-pack round dial panel which included a CDI, DME and ADF. We flew from Kemble down to Exeter for a pre-booked training slot, intending to fly and go-around from three approaches, without any intermediate holds.
- DME Arc and ILS
- ILS procedure outbound from the NDB
- RNP approach
There were strong north easterly winds, so the airport was unusually on runway 08. Exeter has an off airport NDB about four miles to the east which adds some complexity and can make situational awareness more difficult. Remember, the student is hand-flying the aircraft (no autopilot) with foggles on and so will have less bandwidth available to revise and consider the navigation and procedures. This is quite demanding to do in the real world, so worth practicing in a simulator first.
As an added twist, the UK CAA recently announced that GPS fix substitution is permitted for instrument approaches except for the segment between the final approach fix and the missed approach point. It can also be used to fly DME arcs and allows aircraft not equipped with ADF to fly ILS approaches that would otherwise require it. I’ve highlighted the relevant parts of the approach track that can be flown using GPS (in magenta) and ILS (in red).
We were slightly early for our slot, so flew a couple of GPS holds at MULIT which came in as good practice when asked to do some later. Having obtained the Exeter ATIS well in advance and pre-briefed the plate, we called Exeter Radar and gave our training requirements. We were issued a squawk and cleared to fly the DME arc for ILS 08.
Flying the DME Arc
The plate indicates that you should be positioning yourself to track towards the EX NDB on the 177 inbound radial. You can do this in one of two ways:
- Using the ADF, which requires flying a drift corrected heading where the sum of the DI heading and ADF offset results in exactly 177. This is quite tricky to do, especially with winds at altitude well over 30 knots.
- Using the GPS in OBS mode, effectively making the EX waypoint into a “Virtual VOR” and allowing a magenta line to be drawn onscreen for the desired inbound track. This can be tracked using the CDI with simple left/right indications.
However, the DME is located at the airport rather than at the NDB, so you would tune in the DME to the IET frequency and ident it. This will be different from the distance shown on the GPS which would be towards EX, so if not DME equipped and using the GPS exclusively to fly this arc then its important to switch to IET on it at a suitable time (perhaps around16-18 miles out when well established on the inbound radial). This can be done by loading and activating the ILS procedure in the GPS, selecting VECTORS as the initial approach fix since no overlay for the DME arc is provided.
Flying the arc involves setting a series of drift corrected headings that allow you to remain within half a mile of the target range. As your distance increases, you make a significant 10 or 20 degree turn that will bring you closer and subsequently out again. It’s a bit like flying a track shaped like an old threepenny piece and there shouldn’t be very many course adjustments. Decide on your new heading, set the bug and then fly it.
When established on the arc, I would complete the pre-landing checks (which VFR pilots might know as downwind checks) to get them out of the way before it becomes busy. These vary slightly by aircraft but generally include brake pressure check, mixture to rich, turning on the fuel pump, landing light, and double checking the altimeter setting, door and seatbelts. Rate One uses the acronym BUFFPEARL but there are others in common use.
About half way around the arc we were asked to orbit for separation from an inbound executive jet and later given a specific vector for separation. The operation resembled orbits downwind in the circuit and added some further pressure but we recovered the arc distance without losing situational awareness or altitude and continued on.
The question then arises as to what altitude you should be flying at. We had been cleared for the procedure so were permitted (and expected) to descend as described on the plate. Careful inspection reveals MSA which from the circular diagram on the top right of the chart translates to 3100 feet. Only after passing the 094 radial to EX NDB should we descend to 2200 feet. Again this makes it a bit tricky if you had only the GPS tracking the airport, however 094 towards the airport would be reached later and I would suggest acceptable to descend at that point.
Sometime during the latter part of the DME arc it is important to identify the ILS to check it’s working (and that you have tuned in the correct frequency). For older equipment such as the 430, this means listening to the morse code audio. Later equipment such as a GTN650 does that for you and displays the morse code received in text form.
#1 Flying the ILS Approach
The turn onto the final approach track should commence from the 085 radial to EX rather than the airport, but at that point the difference is going to be small. You have a choice here of using the ILS to intercept and track inbound towards the final approach fix or of using the GPS to do so. Since there is no coded DME arc in the GPS database, my preference would be to use the ILS. Nonetheless, we did load the GPS overlay procedure and have this displayed on the screen while the CDI was being driven by the ILS.
It’s quite easy at this point to overlook switching the 430 box from GPS to VLOC mode. This connects the CDI display to the ILS signal rather than the GPS, and so should also enable the glideslope.
It seemed a relatively short time with quite a lot going on – descending to the platform altitude of 2200, intercepting the localiser, reporting this and switching to tower, preparing to capture the glideslope and performing crosschecks at the final approach fix (it helps to verify that the altitude and DME and the glideslope/track needles are all aligned), reconfigure for the appropriate descent rate dependent on groundspeed. To make matters worse, it was quite turbulent over the hills to the west of the airfield and the crosswind component varied considerably as we descended. Nonetheless, we held the track and glidepath within limits down to IRR minima (500 feet above ground = 600 QNH) and went around.
This is where I would have expected the GPS overlay procedure to come into its own. The published missed approach is to fly straight ahead towards the EX NDB and enter the hold. By reverting to GPS mode on the box and pressing SUSP (the suspend button, misnamed because it actually unsuspends/unpauses the GPS navigation sequence), the CDI should continue to track an accurate and relevant path. However, this was for the published missed approach which we weren’t intending to fly. Instead, we wanted to go straight into the ILS procedure which involved flying direct to the NDB then circling around to fly immediately outbound on a reverse course.
#2 Flying the ILS Procedure
That involved some quick button pressing to load and activate the correct ILS procedure. This is one area which I have always felt was weak on the Garmin equipment. I’m unclear whether this is due to Jeppesen (who provide the database) or Garmin (who display it), but the menu option is a list of identical initial approach fixes for each approach. These might differ based on aircraft category (speed) or other factors, but you just can’t tell. Where there are two, then the mantra is “lowest is slowest” and you can be fairly certain that the Cat A/B option is the bottom of the list. Commercial airline pilots strongly encourage visual sanity checking of any approach loaded, ensuring it starts and proceeds on the track you are expecting.
We managed to select, load and verify the correct ILS overlay approach in the GPS before reaching the NDB, also referred to as “the beacon” when reporting position to ATC. Thereafter it sequenced us through the turn back outbound, initially right onto 107 then left onto 250. For those used to ILS procedures, it would seem natural to start a descent as soon as you pass outbound the beacon. However since EX is located some miles east of the airport, you have to wait until west of it. Again, the DME might confuse you because its based on the airport and not at the NDB.
What’s worse is that it will display 2 miles BEFORE you get to the airport, count down further to around 0.6 and then start increasing. Easy to overlook. This is perhaps one feature where GPS sequencing has the upper hand and is easier to follow. Once again, outbound pre-landing checks should be completed after passing outbound the beacon and the ILS should be identified before turning to intercept.
This time, we made more use of the GPS overlay and were able to compare the scaling of the CDI deflection needle as it switched from GPS to VLOC mode. The second time around the procedure didn’t seem quite as rushed, with a good intercept, radio call and checks complete but the wind continued to buffet us at times making it quite hard to stay within limits.
Our missed approach this time was a turn to the north climbing 5000 feet, subsequently towards SISRI. One of the constraints of approaching an RNP initial approach fix is that you aren’t supposed to turn more than 110 degrees onto it. The layout rarely requires this in normal flight operations because you pick the IAF to suit your inbound direction, but during training we can easily find ourselves approaching at almost 135 degrees directly back from the airfield. So it helps to fly north for a bit, sort yourself out before loading and activating the approach. You can always continue north for a bit further and reactivate the approach which will redraw the magenta line from your current location – just press Direct-To, Enter, Enter.
#3 Flying the RNP approach
We were continuing our climb towards 5,000 feet when we were cleared for the approach. This meant we could stop our climb immediately and select a descent rate that would get us back down to 3,400 feet by SISRI. Before reaching, we were asked to hold at SISRI to accommodate other aircraft, but not constrained with any particular direction. This allowed us to pick an easier into-wind radial and choose our own hold entry. Just one hold was enough delay to allow us to be cleared for the approach.
After the complexity of previous approaches with differences between ADF, DME, ILS and GPS modes, the RNP sequence seemed quite easy for follow. My student correctly remembered to check the GPS annunciation mode once established on the Final Approach Track. For RNP approaches, you report to the controller that you have established final approach track rather than localiser established. This mode is not displayed until that stage and would be one of LPV, LNAV+V, LNAV/V or LNAV. There are no LPV approaches in the UK at this time. If you have a WAAS equipped navigation box then it will most likely offer either LNAV+V or LNAV/V. Very few GA aircraft have approval for LNAV/V which has a lower minima marked on the plate, even if the box annunciates it, so always choose LNAV minima in the UK.
The LNAV+V is classed as a non-precision approach even though an advisory glideslope is displayed, so the IRR pilot must apply a minimum of at least 600 feet above ground level making 700 feet altitude the lowest descended to before becoming visual with the runway. Always use the LNAV minima unless your aircraft has explicit approval for something better, which will be stated in a Pilot Operating Handbook supplement.
During the initial approach, the strong winds made it seem quite likely that a landing would not be advisable but as we descended these reduced and I thought a safe landing would be quite feasible. However we had pre-arranged to divert to Dunkeswell so went around and departed as planned.
Not all students would be up to this level of intense workout so I was pleased with the efficient progress we had made, initially through remote Zoom sessions and subsequently in the aircraft. My view is that if a student can’t talk through a sequence of actions and checks on the ground before flying, then it’s worth spending more time on the ground to get that well understood before taking off. Groundschool is much cheaper than flight time, and the aircraft isn’t a great classroom to introduce complex techniques.
Our flight time was 2 hours 30 minutes block time of which 2 hours 20 were airborne.
I regularly instruct two days a week at Gloucester with the Bristol Aero Club, two days at Brize Flying Club (mostly military services) and am also available by arrangement for instruction in student’s own aircraft including for the CBIR through the Rate One ATO.