After mastering VOR tracking in my last lesson it was now time to master ADF tracking. ADF stands for Automatic Direction Finder and it’s device in aircraft used to track NDBs, which in turn stands for Non-Directional Beacon. Once that’s done it was time to try my first instrument approach… but first things first:
This post contains a truckload of ‘published’ information like examples of approach plates or how minimum descent altitudes are calculated. Those plates, values and formulas are for reference only and should not be taken at face value since they might already be out of date. This blog represents my experience and learning process and is not intended to be any sort of official guideline. Please refer to the CAA AIP and/or your flight instructor for the official documents and procedures!
The title picture of this blog post shows a typical ADF (taken from a Cessna 152, pointing to Coventry, of all places). It’s basically made of a frequency selector and a needle pointing in the direction of the NDB.
Since the needle always points in the relative direction of the NDB, intercepting and tracking works a bit differently than on the VOR. The basic principle remains: fly parallel to figure out whether you’re left or right of track, then turn on a heading to intercept the radial, then turn towards the radial. Since the ADF always points in the relative direction of the NDB, and if the aircraft is intercepting at a 45 degree angle, the radial is reached when the ADF points exactly 45 degrees to the left or right depending on intercept angle.
Tracking works similar. As soon as the needle deviates left or right from zero, change heading towards left or right until the radial is intercepted again, then turn towards the radial. Not too complicated, is it? Well if only there was no other work to do than that…
We started the exercise by intercepting the 030 radial inbound to the Gloucester NDB (GST) and flying down at a heading of 210 respectively, without any wind correction (WC). This was to demonstrate that without correction for crosswinds, the aeroplane would be blown off track. After intercepting the radial again we applied WC and huzzah!, we stayed on the desired track.
We asked Gloucester ATC if they would allow us to fly overhead and then track back to the beacon and then outbound on a northerly heading. Luckily they were cooperative (Gloucester ATC are a friendly bunch) and I was told to intercept and track down the 030 radial all the way down to the airport, then a few more miles southwards before doing a left turn back north, fly over the airport and track northbound. Once passed the airport Jerry positioned the aircraft off track just as an exercise to intercept the 360 radial again.
After that Jerry gave me vectors back to Wellesbourne, time for a coffee. This exercise was really just a warm-up compared to what came next!
This is where the whole instrument flying thing gets interesting. From what I gathered so far, instrument approaches are where shit gets real! During an approach the pilot has to fly on instruments, track beacons and descent safely while paying attention and talking to ATC all the time.
According to the FAA, an instrument approach is “a series of predetermined manoeuvres for the orderly transfer of an aircraft under instrument flight conditions from the beginning of the initial approach to a landing or to a point from which a landing may be made visually.” The series of predetermined manoeuvres are published in instrument approach plates and can be downloaded from the NATS AIP pages. A typical approach plate looks like this:
The approach plate gives the pilot information about MSA, the Minimum Safety Altitude in certain sectors within the vicinity of the aerodrome, frequencies of radio beacons and ATC, and a horizontal and vertical view of the approach, the Obstacle Clearance Altitude OCA, and the Missed Approach Procedure (MAP), plus more information which I’m not going to list here.
Approach Procedure Segments
An instrument approach consists of 5 segments: the arrival, initial, intermediate, final and missed approach segments. Here’s what they mean:
- Arrival: what it says on the tin. Arriving at the Initial Approach Fix (IAF). Apparently if the aircraft arrives from an airway there are even published arrival procedures, but that’s of no concern for an IR(R) holder.
- Initial: starts at the IAF and begins the procedure. This is also often the place where the holding pattern takes place. The initial segment ends at the Intermediate Fix (IF). In the procedure above this is at 8 DME on the 081 radial on the Gloucester NDB.
- Intermediate: once at the IF the procedure usually sets the path up for the final approach course. This segment ends at either the Final Approach Fix (FAF) if there is a designated beacon or at the Final Approach Point (FAP) if it is a fix, e.g., 5 DME on the 081 radial of the NDB (final approach track of 261 degrees), as shown above.
- Final: This is where the pilot configures the aircraft for a descent down to the decision altitude, more on that below. For non-precision approaches like NDB a descent to the Minimum Descent Altitude (MDA) can be performed, and then flown down to the Missed Approach Point (MAP), but a constant descent rate is usually preferred.
- Missed Approach: if the MAP is reached and the pilot is not visual with the runway, a missed approach is performed. Missed approach procedures are part of the approach plate and have to be followed unless instructed otherwise by ATC. It usually prescribes a heading and altitude to take on before returning to the IAF to start the procedure again.
In case that the IAF is reached, but a clearance for the procedure is still outstanding, the aircraft must remain in the hold. A hold is based on a fix, in Gloucester this is the IAF itself. The holding pattern is shown on the approach plate, including the direction (left or right) and the tracks to be flown outbound and inbound.
How to enter the hold is defined by three sectors: sector 1 for a parallel entry, sector 2 for a offset (or ‘teardrop’) entry and sector 3 for a direct entry, as shown in the image below:
Once the aircraft is at the holding fix, a 180 degree rate-1 turn must be flown, which takes one minute. After that a 1-minute leg will be flown in the outbound direction away from the beacon, followed by another 180 degree turn back towards the beacon.
It is important to consider the wind for a holding pattern in order to arrive back at the holding fix. Generally speaking the pilot should add around 2 seconds for each knot of headwind (subtract in case of tailwind) for the straight legs. In addition a wind-correction angle is required to remain on track: since the wind factor cannot be added during turns it will instead be added multiplied by 3 on the outbound leg, to cover for the leg and both turns. One the inbound leg the WC is applied only once to arrive back at the beacon.
The Decision Altitude and Minimum Descent Altitude
This topic causes a lot of confusion about what is required and what is recommended for an IR(R) holder, and I don’t claim full knowledge of this topic (yet). For a non-precision approach like the NDB approach, a minimum descent altitude is calculated. Once the descent started at the FAP/FAF, the pilot must not descent below this altitude until either visual with the runway or the MAP is reached. The MDA is calculated as follows: the higher of either the Obstacle Clearance Height (as shown on the approach plate) or the system minimum (for NDBs: 300ft), plus 50ft Pressure Error Correction (PEC) for precision approaches, plus 200ft recommendation for IR(R) holders. If by this point the MDA is lower than 500ft (for precision approaches) or 600ft (non-precision), than use the higher value. Finally, add the runway threshold elevation (for precision approaches) or the aerodrome elevation (non-precision) to the MDA for the final result. Sounds bloody complicated!
Here’s an example: the approach plate above states a OCH of 513ft for Class A aircraft (which includes light aircraft like PA-28s). For the NDB approach we don’t need to add the 50ft PEC, so we stick with 513ft. Next comes the IR(R) recommendation of another 200ft, so we end up at 713ft. Since we’re higher than 600ft, we can ignore that (recommended!) minimum. Finally we add the aerodrome elevation of 101ft to get the final MDA of 814ft. That’s it. One more thing though, due to the aircraft having some inertia on the descent it is recommended to add another 15-20ft in case a missed approach must be executed, to prevent an unwanted descent below the MDA. For our exercise we went with an absolute final value of 840ft altitude.
By this point my brain felt like a smashed old potato. Nevertheless it was time to put the theory into practice. Sort of.
Luckily, Jerry announced that he would do all the radio work. That was good, because there was little talk about how to communicate all the steps with ATC and thus I had no clue what’s going to happen in that regard.
Initially we decided to practice the approach around a ‘fake’ NDB station, for which we chose the Radio5 antenna near Droitwich Spa. Radio antennas operate in the same frequency band as NDBs, so they can be used for aerial navigation, but really shouldn’t! After flying past the beacon (the IAF) we lost contact to the radio station, so we decided to go south and ask Gloucester Airport if they could squeeze us in for a real NDB approach. Luckily they said yes.
Heading south we tracked down the GST NDB at 4,000ft to arrive at the IAF. Jerry explained to me how to enter the holding pattern, in this case by using the ‘parallel sector entry’, and announced that we were ‘taking up the hold’. In the hold it is important to calculate a leg time and apply appropriate wind correction. Since we had some headwind we flew the leg for 80 seconds instead of the usual 60 seconds. In the hold we were cleared by ATC for the NDB approach.
Back at the beacon we tracked the 092 radial from the beacon and descended to the platform altitude of 2,300ft as given by the approach plate. Jerry announced that we were ‘beacon outbound’, and we were asked to report ‘base turn complete’. This is a good point to perform all necessary pre-landing checks. After reaching 8 DME we started the base turn to intercept the 081 radial inbound and started a small descent down to 2,000ft.
The final segment would start at 5 DME, so just before that we started to slow the aircraft down, set 10 degrees of flaps and trimmed for a stable 400ft/minute descent, still tracking towards the beacon on the 081 radial. The approach plate gives a table of altitudes at which the aircraft should be at certain distances, and we were surprisingly accurate. We were ‘cleared for the low approach’. Now that we were getting closer to the MDA Jerry mentioned that the ADF was not exactly pointing in the right direction and thus we’ve been a good deal off track, but he repositioned the aircraft just before MDA to give me an idea what it should look like.
Once we reached the MDA we decided that we couldn’t see the runway and went around for the standard missed approach procedure, turning onto a heading of 357 and climbing back to 2,800ft, and shortly after went back to Wellesbourne.
ADF tracking is not as easy as VOR tracking because of the information given by the instruments, but all in all I had little problems understanding the concept. As usual I had a little tendency to over-correct stuff, but nothing that wouldn’t go away with a little practice.
As for the NDB approach, there was a lot of theory to take in before I could go and execute the approach. I didn’t understand all at once (by the time I write this I have a much better albeit not-even-close-to-perfect understanding), but I got the basics. In general the flying went ok, but I often had no clue where in the procedure we’ve been and I just followed Jerry’s instructions. Instrument approaches seem to be one of those things that must be permanently practiced. One recommendation Jerry gave me was to perform a IFR flight at least once a month to stay current!
Now the rest of my training will be a lot more instrument approaches, and I can already see that this will require quite a few more attempts to get right, but I’m ready for the challenge!
PS: I’m still a student of instrument flying and therefore I probably missed a million points or got stuff wrong. In that case feel free to comment, (positive) feedback is always more than welcome!
Cheers and keep on rockin’ the skies!
Course Time: 8:55h
Under the Hood: 7:35h