Nearly all of the time, we end up landing out of a "for-real" instrument approach. An instrument approach gets us down pretty far — around 500 AGL for a non-precision approach, and 200 AGL for a run-of-the-mill ILS. It’s not very often that we’re not out of the clouds at minimums.
If we’re not, the weather is pretty rotten, and we should know about it ahead of time, from weather forecasts and weather reports from either the destination field itself, or a nearby (usually larger) airport.
So, most of the time we’re pretty certain that we won’t have to execute the missed approach. But every once in a while it does happen.
A few months ago, I was sitting on the ground one morning at Thunder Bay. The airport was covered with a thick layer of radiation fog that formed overnight. It was clear above, but RVR 600 feet underneath.
So, all of the departing aircraft were waiting for the visibility to pick up to a half mile (RVR 2600) so we could legally take off. After a while, sure enough, the ATIS reported the RVR wobbling up and down, and the bottom of the cloud layer rose to 100, then 200 AGL.
Inbound aircraft started making approaches. Several aircraft made it in, flying the ILS 07 to minimums. Then one guy called the missed approach. He apologized, and said that he didn’t see anything at 200 feet. It happens.
But it shouldn’t have been that much of a surprise in this case.
Flying the Missed
When I’m looking over an instrument approach, I don’t try to memorize the missed approach instructions.
One thing is for certain: every miss begins with a climb. Try to find one that doesn’t. What’s important to know is if you climb straight ahead — which is easiest, just hold your heading — or if you have to make an immediate climbing turn, usually to avoid obstacles. If you simply memorize that much, you can look up the rest as it’s needed.
The important thing in a miss is to get a good climb angle started immediately. Full power, gear and flaps up.
It’s a busy time. A few hundred feet above the ground is no place to be trying to find the missed approach instructions on the plate. You need to know ahead of time — all you need to know, ahead of time — is whether you initially climb straight ahead, or if you need to be turning left or right during the climb.
Once you have the aircraft configured in the climb, then you can get your head down, and get set up for the navigation portion of the miss.
DH vs. MDA
Every once in a while, some confusion arises between the decision height (DH) of a precision approach, and the minimum descent altitude (MDA) of a non-precision approach.
Let’s take a look at the definition of MDA:
A specified altitude referenced to sea level for a non-precision approach below which descent must not be made until the required visual reference to continue the approach to land has been established.
It’s pretty clear that we’re not supposed to go below the published MDA on a non-precision approach, unless we can see enough to descend and land.
However, some people get the idea that the same is true for the decision height of a precision approach, which has glideslope information. They want to level off, and not descend below the published DH.
This is not a good technique. Established on the localizer and glideslope, a stablized descending approach is the goal.
Let’s take a look at how DH is defined:
A specified height at which a missed approach must be initiated during a precision approach if the visual reference to continue the approach to land has not been established.
The definition quite carefully does not say that we cannot descend below DH. It says that if we’re still in the murk at the DH, we should put the power on and start the miss.
But if we’re in a stabilized approach (and descent) at DH, our momentum will naturally carry us slightly below DH during the start of the miss. The larger the aircraft, the more inertia it will have. Urban legends abound concerning older jets (like the Boeing 707) actually contacting the runway after a missed approach was started. Some of them may even be true.
Non-Precision Approaches to Minimums
Back to non-precision approaches, which are defined as not having any glide path information. If the weather at the destination airport is good — cloud bases are reasonably high, and the visibility beneath is good — we’ll expect to see the airport quite a way back. This will make the visual descent and maneuvering to land pretty easy.
But if the weather is near minimums, we need to think a bit more carefully about the visual descent from MDA.
The approach plates tell you how to locate the missed approach point (MAP), usually with a timer. Look at any non-precision approach.
You’ll notice in the profile that the MAP is almost always located above the runway. That means if we’re trucking along at the MDA in cloud, and suddenly see the runway at the MAP, we’re not likely in a position to land from 500 feet up, unless the runway is very very long, or we have parachutes, and are willing to use them!
What I do is subtract a minute from the MAP time, and I expect to be visual with the runway by that time. Otherwise, I know I’m going to have to miss, and I’m just marking time until the MAP.
An MDA is typically around 500 AGL, so if we’re visual with the runway a minute before the MAP, we have one minute to lose 500 feet, which results in a sedate 500 fpm descent.
Note that at 90K groundspeed in a single-engine airplane, this means that we will need a minimum of 1.5 nm visibility to be visual at least one minute from the MAP. If you’re flying a twin with a 120K approach speed, make that 2 nm.
If we are visual at the MDA 30 seconds from the MAP, that means we will need a descent rate of 1,000 fpm to make the runway, which is tough. At 90K, that means we have less than 1 mile visibility.
For a non-precision approach, I really like to have 2 miles visibility. When people start talking about visibilities of 1 mile or less, I start looking for an ILS.