In this discussion I will assume we fly an instrument approach on autopilot, in other words coupled in a Cirrus Perspective aircraft. The question in the title is more specifically how to stop the descent at Minimum Descent Altitude, while flying coupled. The article assumes working knowledge of avionics. It applies to all Cirrus SRXX Perspective or Perspective+ models. Answers to quiz questions are at the end.

To make it simpler, let’s state from the beginning that we will ignore Vertical Navigation (VNV) and this is because it can only bring an airplane to the Final Approach Fix. VNV disappears before FAF on Perspective+ and a fix preceding FAF on Perspective.

Precision approaches

Precision approaches are simple. You intercept glideslope or glidepath, descend to Decision Altitude, look for runway environment, disconnect autopilot and land, or press TOGA to initiate missed.

Quiz questions:

  1. What is the difference between a glideslope and a glidepath?
  2. Is an approach with a glidepath always a precision approach?
  3. How about one with a glideslope, is it always a precision approach?

Once you started descending on glideslope or glidepath (GS/GP), you should enter the missed approach level off altitude in the altitude selector. That is because once the GS/GP is active, the airplane will fly through an altitude in the altitude selector. In all other autopilot modes, the airplane will always capture altitude entered in the altitude selector, but not when on active GS/GP, in that mode altitude selector is ignored. If you didn’t know that you should observe that except on active GS/GP, ALTS is present in the autopilot scoreboard, confirming it is armed and capture will occur. Not so when GS/GP is active.

ALTS is absent when GS/GP is armed

Non-precision approaches

Non-precision approaches present more options, we will start with a mother of them all, a localizer approach. On a localizer approach, there is no glideslope, so the only option is to descend in VS mode. Since altitude selector is now armed, we can enter MDA there and the autopilot will level off. Score 1 for entering MDA into altitude selector.

How about the other very popular non-precision approach, LNAV? Before we continue, let’s clarify that LNAV is not a type of an approach, it is Level of Service for a RNAV approach. The title on an approach chart (in US) will be RNAV (GPS) or just GPS and the chart will depict different minima for different LOS, such as LNAV, LP, LPV or LNAV/VNAV. However, for the sake of simplicity, I will call them here different approach types.

You might be tempted to say, let’s do the same, LNAV is a non-precision approach with a MDA like a localizer, let’s enter MDA in the altitude selector and descend on VS. That is indeed one possible way, but it ignores the fact that most LNAV approaches do have glidepath and the avionics annunciates them as LNAV+V. This is a so-called advisory glidepath.

Another quiz question:

4. Do all LNAV or LP procedures always have advisory glidepath, i.e. be annunciated as LNAV+V or LP+V?

Advisory glidepaths are generated by avionics, FAA knows nothing about them, and approach charts never reference them. Nevertheless, it is a glidepath and the autopilot will couple to it in exactly the same way as to a glidepath on a LPV approach. Looking at the Vertical Deviation Indicator, you will in both cases see a solid magenta diamond and wouldn’t be able to tell looking only at the VDI if you fly LNAV+V, LP+V or LPV. Consequently, if you intercept that glidepath and descend on it, anything in the altitude selector will be ignored.

Now we have two options:

  • Dive Option: Do not arm intercept, descend on VS and enter MDA in the altitude selector. This truly uses the glidepath in an advisory-only mode, we can change the rate of descend to follow it, but we don’t couple to it.
  • Glide Option. Arm the intercept and couple to the glidepath, but now we must figure out how to stop the descent at the MDA. The only way to do that when coupled to GS/GP is to press ALT when at MDA .

Dive Option seems to give up on a nice part of automation, smooth descent coupled to a glidepath. Furthermore, you now have two different scenario when being cleared for an approach. For a precision approach, you would press APR button arming GS/GP intercept, on a non-precision approaches you would not.  It is always preferable to have simple procedures that always work the same, so having that bifurcation is not ideal.

Glide Option preserves the advantages of using the automation to descent on glideslope and preserves the same procedure of arming approach by pressing APR button when cleared for an approach.  Flying with a finger on ALT button and waiting for the airplane to descend to MDA before pressing it seems silly though.

How can we get around stopping descent with ATL? By decoupling from GS/GP above MDA. When 100′ above press NAV button, this will revert the autopilot to PIT mode and enable ALTS capturing the altitude in the altitude selector. We will consider this modified procedure moving forward.

Vertical Descent Point (VDP)

Before making decision between glide and slide options, let’s consider other factors. Any straight-in non-precision approach should have a Visual Descent Point. Many pilots remember that if they arrive at VDP and don’t have runway environment in sight, they should initiate missed approach, because they will not be able to descent and land using normal maneuvers if they fly past VDP. Speaking simply, they will be too high. We should also remember that AIM 5-4-5 says “VDP is a defined point on the final approach course of a non-precision straight-in approach procedure from which a stabilized visual descent from the MDA to the runway touchdown point may be commenced. The pilot should not descend below MDA prior to reaching VDP.” Descending below MDA prior to a charted VDP may result in encountering difficult to see obstacles. If a VDP is not charted, a pilot would be well advised to compute it, since by definition, flying past it does not allow to descent for landing using stabilized approach criteria. For example, the chart above is from LOC 28L MRY with MDA at 1603’ AGL. We need 2.5 minutes to fly 4.2 nm from VDP to MAP at 100 knots, and 640 fpm descent. Flying past VDP would mean even faster required descent rates.

However, notice that if there is an advisory +V glidepath, it will intersect MDA approximately at VDP, we can keep descending on the glidepath and when we arrive at MDA, we either see the runway and land or we don’t and we go missed. In other words, we can fly any approach with GS/GP the same way and arriving at MDA or DA, we land or go missed using TOGA.

Circling approaches

Flying a circling approach, we may or may not have a GS/GP, but we still need to level off at the MDA and in most cases, we are not to immediately start a missed approach, as is the case with straight-in approaches when arriving at VDP. In fact, the concept of VDP does not even apply to circling approaches. Well, it sort of does, but that’s a subject for a different article.

Dive Option

  1. If there is a DA, descend on GS/GP
    • Arm GS/GP by pressing APR when cleared for approach
    • Put altitude of missed in the altitude selector
    • Descend on GS/GP
    • At DA, either land (disconnect autopilot) or go missed (TOGA)
  2. If there is MDA
    • Do not arm GS/GP intercept, press NAV as necessary, but not APR
    • Put MDA in the altitude selector
    • Descend on VS (or faster) using GS/GP as advisory tool if present
    • At MDA, level off and fly to VDP or MAP

Glide Option

  1. If there is a DA, descend on GS/GP (same as dive)
    • Arm GS/GP by pressing APR when cleared for approach
    • Put altitude of missed in the altitude selector
    • Descend on GS/GP
    • At DA, either land (disconnect autopilot) or go missed (TOGA)
  2. If there is MDA
    • Put MDA in the altitude selector
    • If there is no GS/GP, descend on VS
    • If there is GS/GP, arm it and descend on it. 100′ above MDA disable GS/GP by pressing NAV.

I admit that I initially liked more Glide Option. Ignoring GS/GP when present seemed a weird proposition. I sent a draft of this article to Peter King and his invaluable feedback made me think. Here are some of his arguments:

When the weather is low, it is preferable to descend to MDA and level off before VDP (straight in) or MDA (circling), because you break out earlier and thus have increased chances of seeing the airport.

Regarding pressing APR when cleared for an approach, that can’t be absolute neither. Peter wrote: That blanket guidance has some shortcomings. For example, what do you do when you are being vectored to an arc on an ILS, or when cleared for a localizer-only variant of an ILS because the glide slope is NOTAMed OTS but the glideslope signal is still present and may or may not be right?

I tried the dive option for the while and realized that it does in fact complicate the matter somewhat – it is therefore controversial for initial instrument training. It ended being up a matter of personal preference.

Which option do you prefer?


1. What is the difference between glideslope and glidepath?

Glideslope is a radio signal, it is generated by a ground transmitter and depicted on VDI with a solid green diamond. Glidepath is generated by airplane avionics based on WAAS GPS data and is depicted with a solid magenta diamond.

2. Is an approach with glidepath always a precision approach?

No, an advisory glidepath is displayed on LNAV and LP non-precision approaches. While advisory in nature, it is indistinguishable from a real glidepath on the display and is used in the same way by the autopilot. Pilot should remember that advisory glidepath does not provide obstacle protection below MDA.

3. How about one with glideslope, is it always a precision approach?

Although very rare, there are also LDA (non-precision) approaches with glideslope, for example LDA 25 EGE.

4. Will all LNAV or LP procedures always have advisory glidepath, i.e. be annunciated as LNAV+V or LP+V?

If an approach has only circling minima, advisory glidepath will be unavailable, for example RNAV A MEV. Your navigator must support WAAS to get +V, if you have the original GNS430 (not upgraded to GNS430W), you will not see it.