Updated: Dec 8, 2020
Instrument Approach Procedures Lesson by wifiCFI
What is an Instrument Approach?
An instrument approach is a way to safely get a pilot lined up with a final approach and landing to a runway.
It guides the pilot from the en-route or arrival phases of flight to approach and landing at the destination.
In this lesson we will discuss:
How to read Instrument Approach Plates
How to fly an ILS approach to landing
Requirements to descend below Instrument Approach Minimums
Understanding the Instrument Approach Plate
See the wifiCFI Lesson for Approach Chart Reading and Symbology.
Minimum Sector Altitudes (MSA’s) are given to pilots for emergency situational awareness.
The MSA will always be established for a certain radius around a point (typically a navigation system)
In this example we can see the MSA is established as a 25NM radius around the OGD VOR.
The radius is then broken into sectors and each sector is assigned an altitude.
If flying within the established radius and at the altitude assigned for the sector, a pilot is guaranteed 1,000’ Obstacle Clearance in Mountainous and Non-Mountainous areas.
VDP stands for “Visual Descent Point.”
The VDP allows a pilot ample time to see the runway environment and establish a normal (safe) descent path to the runway.
Therefore, if the pilot does not have the proper runway environment criteria in sight at the VDP, he/she knows that a Missed Approach Procedure will be necessary.
However, the pilot must not commence the Missed Approach Procedure at the VDP, but rather at the designated MAP (Missed Approach Point).
VDP’s only apply to non-precision approaches.
Calculating the VDP
Some instrument approaches have published VDP’s while others do not.
If an approach does not have a published VDP, a pilot can figure his/her VDP using a 3* Descent Angle.
The pilot will take the HAT (Height Above Touchdown)/300 = VDP Distance from end of Runway.
In the example below, we can see the HAT is 507’.
We will take 507/300 to calculate our VDP.
In this example our VDP would be 1.7 NM from the End of the Runway.
Aircraft Approach Categories
How to know which Aircraft Approach Category Minimums a pilot should adhere to…
Approach Categories are based on an aircraft’s approach speed.
Some aircraft will have published approach speeds in the aircraft’s POH.
Some aircraft will not have published approach speeds in the POH.
In this situation, a pilot can figure his/her approach speed by taking the aircraft’s Vso (stall speed) and multiply it by 1.3.
Once the pilot knows his/her aircraft’s approach speed, he/she can also know which approach category they fall in by adhering to the approach category numbers below.
The pilot will then use the minimum altitude and visibility values assigned to his/her approach category.
For instrument approaches, there are 2 types of minimum altitudes:
DA (Decision Altitude) – Used on Precision Approaches.
DH (Decision Height) – Same as Decision Altitude but given in an AGL Altitude rather than an MSL Altitude.
MDA (Minimum Descent Altitude) – Used on Non-Precision Approaches.
MDH (Minimum Descent Height) – Same as Minimum Descent Altitude but given in an AGL Altitude rather than an MSL Altitude.
Circling Approach Minimums
Circling Minimums are established so pilots can fly an approach to a particular runway but then land on another runway.
Take this example, our pilot executes an Instrument Approach to Runway 9 but the winds at the airport are reporting 270 @ 15 knots.
The pilot can execute the approach for Runway 9 and then circle to land on Runway 27, which is favorable for the current wind conditions.
Each different aircraft will have a different minimum “flight visibility” based on it’s approach category and the type of approach being flown.
What is the definition of “flight visibility?”
It is the horizontal distance the pilot can see from the pilot seat in the cockpit.
This means it is based on pilot judgement not reported ground visibility values found on ATIS/AWOS reports.
Descending Below the DA or MDA (FAR 91.175)
In order for a pilot to descend below the DA or MDA published on the Instrument Approach Plate, he/she must meet the criteria listed below.
If this criteria is not met upon reaching the DA or MAP the pilot must immediately execute the published Missed Approach Procedure.
1: The aircraft is in a position to make a safe landing using normal descent rates and maneuvers.
2: The flight visibility is not less than what is prescribed on the Instrument Approach Chart.
3: The pilot must have one of the following in sight:
A: The Threshold
B: The Threshold Markings
C: The Threshold Lights
D: The Runway End Identifier Lights (REIL’s)
E: The Visual Glideslope Indicator
F: The Touchdown Zone
G: The Touchdown Zone Markings
H: The Touchdown Zone Lights
I: The Runway
J: The Runway Markings
K: The Runway Lights
L: The Approach Light System
If the pilot sees only the Approach Light System, he/she may descend to 100’ above the Touchdown Zone Elevation (TDZE).
At which point, the pilot must see one of the other items above OR the red terminating side row bars on the approach light system in order to continue to land.
If the pilot descends to 100’ above the Touchdown Zone Elevation (TDZE) and does not see one of the other criteria listed above, then he/she must immediately execute the published Missed Approach Procedure.
FAA Sources Used for This Lesson
Federal Aviation Regulations Part 91
Instrument Flying Handbook