Odd Ball Approaches

Well now that’s very odd….

strange cessna approach

RWY 24 VOR approach instrument pilot ground school

So you want to fly one of these approaches that say RADAR OR DME Required.  Okay, so what if you’re missing one?  Well, that’s where NOTAMs and taking with ATC comes into play.  Before commencing this approach, it is up to you, the PIC, to determine if at least one of those things is working (by talking with ATC, checking NOTAMs, and tuning in and identifying a signal).  Let’s say the DME is NOTAM’d out, and ATC confirms it is not working, but they confirm that their RADAR is working.  Then you could say:

“Detroit Approach, Cessna 8MA, negative DME, request you RADAR Identify the final approach fix, DIXBO, for us today”

They don’t hear requests like that often, so bear with them a moment, and be ready to repeat or rephrase.  Be SURE that they are understanding the request.  The ATC controller, although a true professional, may suddenly go all “private pilot” on you and say “roger” or some other form of non-acknowledgment because they do not fully understand what you are asking for.  If you doubt they know what you’re talking about, verify it with them, I.E.,

“Detroit Approach confirm you are able to and will RADAR identify and let us know when we are over the DIXBO final approach fix for Cessna 8MA?”

Now I know that certainly doesn’t sound like “standard phraseology” but at that point, who could possibly misunderstand what it is you are asking them to do and when you want them to do it?

“Cessna 8MA, Three miles from final approach fix. Turn left heading zero one zero. Maintain two thousand until established on the localizer. Cleared I−L−S runway three six approach. I will advise when over the fix.”

Once over the FAF

Once you are crossing the final approach fix, expect to hear:

“Cessna 8MA, Over final approach fix. Contact tower one one eight point one.”

Now you’ll want to start that timer because that is the only way you are going to know now when you need to go missed (either that or when ATC calls you back and says you’ve flown 10 miles past the airport and suggest you climb immediately).  Now without a GPS, how do you know what ground speed you are doing?  Well, with excellent pre-flight planning, you should know the winds aloft and based on your indicated airspeed, be able to figure out your ground speed.  Now, depending on how good you are or are not at math, option #2 may be more accurate.  Simply ask ATC what your ground speed is currently showing once you’re slowed down and configured.  Approach control can give you a ground speed that is fairly accurate (usually rounded to the nearest 10 knots, i.e, 60kt, 70kt, 80kt, etc.)  Although the MAP is technically defined by time and not a RADAR fix, that certainly would not stop me from requesting Approach control notify the Tower via the landline (or monitor approach on Com #2), and have Approach relay to Tower, to relay to you, to “Go the F*@K around” if you fly past the runway threshold and still do not see the runway.

Planned Descent Point

Notice this example chart does not have a VDP (visual descent point) marked on it.  Yet there is certainly no chance of you arriving at minimums (629′ agl) and at the threshold of the runway (the MAP), and being able to last minute “break out” of the clouds and descend 629′ and still land on the 3,500′ long RWY.  Thus, it seems logical that if you reach minimums of 1460′ msl (629′ agl) and do not see the runway by the time you reach your calculated PDP (planned descent point), that you begin climbing away from the ground at that point (remember getting more altitude is fine, just don’t make any turns “going missed” before you actually do pass the MAP, this is for terrain avoidance).

So how do you know where the PDP is?  Well since we’re doing this approach with timing for our MAP instead of distance, I’ll show you that way first.

  1. Take your HAT (height above touchdown that the MDA is) in this case: 629′
  2. Divide 629′ by 10.  I like round math, so let’s say 630/10
  3. That equals 63.  In a jet we’d stick with 63 (at 120 ish knots), for our prop planes (60-70kt final approach ground speed, we double our answer, 63 becomes 126
  4. That 126 is the time in seconds it is roughly going to take us to get from 629′ above touch down zone elevation, down onto the runway if we maintain a 3-degree glide path.
  5. This means if we take the 4:42 seconds it takes to go from DIXBO to the MAP, and subtract 126 seconds from it, that is the point where we’d want to think about starting our missed approach climb, then following the missed approach procedure after passing the MAP.
  6. So to sum it up, you are notified you are crossing DIXBO and you start your timer.  2:16 goes by (4:42 – 126 seconds), and you have descended to 1,460′ msl, you don’t see the runway at all.  At this point, you say “okay, if I stay down here I may see it, but I’ll be really high and have to dive for it, that’s a bad idea, so let’s start the climb now knowing we are going to be going missed.”
  7. Following these rules will likely result in a longer instrument flying career that is not abruptly ended by trees or a cell phone tower.

PDP Based on Distance

rnav gps approach plate

Finding your PDP based on Distance from the runway or localizer DME is much easier than doing it based off timing (and you don’t have to worry about your groundspeed either).  To find you PDP based on distance:

  1. Take the HAT. In our example on the right, it is 444′
  2. So 444’/300 = 1.48, round that to 1.5
  3. So 1.5nm from the RWY is your PDP
  4. Note: if the runway threshold was, say, 1.1nm DME on a Localizer, than you would have to add 1.1nm + 1.5nm to get an accurate PDP.

This “PDP” distance is where you need to be leaving MDA to make a nice 3 degree descent to the runway.  If you arrive at MDA. and are at 1.5nm from the RWY, and do not have the RWY in sight, then you can just go ahead and start your climb up to 2,000′ (per the missed approach procedure).