Like most other instrument pilots and instrument students in General Aviation, I must fly instrument approaches with precision… and hope. I must fly the procedure correctly, but I must also hope that the cloud base under me and visibility will be high enough for me to see the runway at the end of the approach. We spend a great deal of time practicing the approaches so that we can get into that position, but what happens after you arrive at the end of the approach?
What are the rules that allow you to fly below the Minimum Descent Altitude (MDA) or Decision Altitude (DA) and land?
The rule actually has three parts for every instrument approach and then some approaches have a fourth part on top of that. This article deals with the first, and most obvious, part of the rule — the ‘runway environment.’
RUNWAY IN SIGHT
The first part of the rule says that at some time during the approach the pilot must be able to look out of the aircraft’s windows and see the runway ahead. This will not happen if the cloud base is lower than the MDA or DA and/or if the visibility is so poor that forward vision is blocked. In that case the pilot must make a missed approach and either try again or fly on to somewhere where the weather is better. This part of the rule was once called ‘runway environment’ but today the regulation [91.175 (c)(3)] specifically defines what constitutes a runway in sight. The first part of the rule says that before a pilot can fly below the MDA or DA for that approach, that he/she must see one of the following items:
- The Threshold (the approach end of the runway)
- The Threshold markings (the 8 while lines painted on the threshold – 4 either side of the runway centerline)
- The Threshold lights (row of lights along the threshold – the lights are green as you approach the runway, but are red on the back side to signal the opposite end of the runway)
- The Runway End Identifier Lights (white flashing strobe lights on each corner of the runway)
- The Visual Approach Slope Indicator (can be any visual approach system including VASI – 2 bar or 3 bar, PAPI, Tri-color system, or Pulsating system)
- The Touchdown zone or its markings (white stripes painted on the runway at 500-foot intervals down the runway. These are grouped with 3, 2, or 1 stripe)
- The Touchdown zone lights (white lights embedded into the runway itself that form two rows on either side of the centerline)
- The runway or its markings (the actual runway surface and/or its markings such as the runway number)
- The runway lights (white lights along the edge of the runway. These lights can be of low, medium, and high intensity. On instrument runways the last 2,000 feet is indicated with yellow instead of white lights)
If you don’t see one of those things, you can’t descend below the MDA or DA to land without declaring an emergency. Remember: The term ‘runway environment’ no longer applies. That language has been removed and replaced with the specific items listed above.
If the runway you are approaching has an approach light system, then the rule [91.175 (c)(3)(i)] allows you to descend below an MDA or DA even before you see any of the nine items on the list above — as long as you see the approach light system. This allows you to ‘fly in over the lights.’
Important: The ‘fly in over the lights’ method does not allow you to land. You can’t come any closer to the runway’s touchdown elevation than 100 feet while over the lights. It is hoped that from 100 feet one of those other nine items will become visible and that will allow you to land. If you fly in over the lights and don’t see any of the nine items, you can’t land.
Note: the airport elevation and a specific runway’s touchdown zone elevation may not be the same because airports are not always completely flat. The pilot must memorize the TDZE before starting the approach, because there will be no time to look it up while flying in over the lights!
VARIATIONS ON THE THEME
There are many variations of approach lighting systems (ALS). Some have the sequenced flashing lights that appear as a ball of light leading you to the runway. These are sometimes called ‘rabbits.’ Prior to flight and certainly prior to the approach the pilot should look up what ALS the runway has and become familiar with its pattern. A row of lights could be a line of streetlights lining a parking lot instead of an approach light system leading to a runway.
…BUT YOU STILL CAN’T LAND
But even if a pilot sees the ALS and all nine items off the list — they still are not allowed to descend to land. There are at least two more stipulations, one of those is the ‘flight visibility’ but we will cover that next week in Part 3 of How IFR pilots get back to Earth.