When Do I Start the Missed Approach During an SCDA?

Most instrument rated pilots are now familiar with the philosophy and procedures for conducting Stabilized Constant Descent Angle (SCDA) approaches – sometimes referred to as a Constant Descent Final Approach (CDFA). The SCDA is a method by which we no longer “step down” on a non-precision approach (NPA); rather we calculate or create a pseudo glide path that respects all minimum altitudes on the approach plate.

The benefits of SCDA are significant as we eliminate multiple power changes, level-offs, and ultimately “chop and drop” events, which have led to many accidents over the years.  This is why on the IFR flight test, you are expected to do an SCDA whenever possible (if you don’t it will be recorded as a major error). If you would like to learn more about SCDA basics before continuing on, check out Advisory Circular 700-028. You can also reference the IFR Flight Test Guide for additional information.

One of the main issues seen by IFR instructors and examiners is confusion from students about when to start the missed approach if the candidate does not establish visual reference with the runway environment. Here is a quick review of where the approach is terminated, and we MUST go-around:

1 – Precision Approach (ILS, LPV, LNAV/VNAV): Start the missed approach when you reach the decision altitude.

2 – Non-Precision Approach (everything else – LNAV, LOC, VOR, etc): Start the missed approach at the missed approach point(DME, GNSS waypoint, radial, time, etc).

With respect to the go-around, keep in mind that it is a pseudo-DA on an SCDA, not a true DA. A true DA is assessed for obstacles such that at the DA, if you don’t have the runway environment in sight and commence an overshoot, the “dip” in altitude you experience below the DA is accounted for as you transition to your climb.

An MDA is assessed only for a LEVEL-OFF, not for any sort of dip. The solution is that on an SCDA with a published MDA, you MUST add some amount of altitude to the MDA to get your pseudo-DA; so that at no point during the transition do you descend below the MDA (the IFR flight test guide recommends a 50’ buffer).

The calculated pseudo-DA and the MAWP generally coincide, and any difference is normally negligible in the real world.  You would for the most part be focused on the pseudo-DA.  However, there are situations where you must be careful to respect the MAWP.

You may find that your vertical planning is off because of wind or other factors (especially if the GPS does not provide you with a vertical deviation indicator). In this case, if high, you may cross the missed approach waypoint (MAWP) significantly PRIOR to reaching your pseudo-DA. Remember, on an NPA you must start the go-around at the MAWP (rule #2 above) – don’t continue to your pseudo-DA if you reach the MAWP unless the runway environment is in sight!

Another situation, though rare, is that in order to produce a constant approach slope upwards from the touchdown zone while respecting the crossing altitudes, the calculations may result in a need to cross the MAWP at an altitude significantly ABOVE the MDA. For example, have a look at the RNAV Z 08 at CYTZ below. The MAWP is DUVUM and the MDA is 680’ ASL:

Now, take a look at what the GPS calculates as a required altitude to cross DUVUM to keep a constant 3.00 degree descent angle:

That’s right, 849’ – you need to cross the MAWP 169’ above the MDA. Be sure to start the missed approach procedure at DUVUM even if you’re still above the published MDA! If you continue without visual reference to the runway environment beyond the MAWP, even if above the MDA, the aircraft is now in an undesired aircraft state and would be in violation of CAR 602.128(3)b.

So why is the SCDA crossing higher than the MAWP MDA?  And why is it a safety issue if you pass the MAWP and continue descending without the runway environment in sight?  It all comes down to obstacle clearance requirements during various segments of the approach and missed approach.  Not commencing the missed approach in the correct location carries a significant risk of controlled flight into terrain.  Though rare, you can run into situations like this example approach… so reviewing your approach charts and procedures in advance is highly recommended!

Of course, there are times when using the SCDA method is not appropriate, such as when circling for another runway or limiting exposure to icing conditions. The advisory circular and flight test guide linked above describe situations where a traditional approach and level off at MDA is the better course of action. That is why non-precision approach plates continue to publish an MDA, and not a DA that is simply 50’ higher. It is up to the pilot to assess the type of approach to be flown, make the necessary calculations, and execute the approach respecting both the MDA and the MAWP.

This article was updated in December 2020 with additional references and clarifications.