All instrument rated pilots will be, or should be very aware of the published minimas for
any given approach and will of course know that one cannot descend below this altitude
without the prescribed visual contact with the airfield. While knowing this and sticking to it
is essential, an understanding of how and why these minima are set will definitely make one a more complete and ultimately safer instrument pilot.
Firstly we need to understand minimas are and what exactly they mean to us in the cockpit.
They are broken down into two categories, depending on the type of approach being flown.
A decision Altitude (DA) applies to a precision approach or an approach providing both
horizontal and vertical guidance(ILS) while a Minimum Descent Altitude (MDA) applies to a non-precision approach providing only horizontal course guidance. The difference between the two is that while on an ILS approach the decision to continue or go-around is made at DA meaning that the aircraft can descend through this altitude while the go-around is initiated while the MDA on a non-precision approach may not be descended below unless the required visual contact is made. Practically this means that one needs to either level off just before the missed approach point or initiate the go-around slightly above MDA to allow for inertia to be overcome and the aircraft to stop descending and then begin to climb. This difference is due to an ILS always being aligned to the runway meaning the initial part of the missed approach always takes place over a runway which is sure to be obstacle free while a non-precision approach is not necessarily aligned with the runway and consequently obstacles may be present.
The next thing to effect minima’s is the navigation aid on which the approach is based, different systems have different system minima based to the accuracy of the system as shown in the table below.
If one adds the ground system tolerance and airborne system tolerance the total allowable
tolerance or deviation from the published track is obtained and we can see this is not
insignificant and can result in a noticeable track error. These inaccuracies result in different
system minima depending on the accuracy of the system.
Facility Lowest MDH
ILS (CAT 1) 200ft
VOR DME 250ft
The greater the error the higher the system minima as the risks associated with being off
track at low altitudes are obvious.
Not only are minima defined by the accuracy of the system providing guidance, they are
also affected by the terrain around the airport. This not only affects the approach but also
the go-around which must be taken into account when the minima are set. The diagram
below shows the required terrain clearance on the go-around.
If we take the NDB A approach for runway 16 at Pietermaritzburg as an example. The
minimum descent altitude is 3000ft (577ft above runway threshold elevation). This is
considerably higher than the system minima of 300ftdue to the terrain on approach as well
as the terrain on the missed approach. Looking at the above diagram we can already see
that the obstacle clearance is not large (98ft) and is based on a climb gradient of 2.5%. The
dangers of straying for example 200ft below minima is not only that the track error may
allow you to stray towards high ground but will degrade obstacle clearance on the go-around and with potentially only 98ft of clearance the dangers are real and obvious.
Add to this turbulent conditions, a high workload and worst case scenario, a single engine go-around in a twin, those few feet may well be the difference between a safe flight and
Note: the above is based on ICAO document 8168 on aircraft operations and as a signatory
to ICAO all published approaches within South Africa will be compliant. The full document is available in the ICAO website and is definitely recommended reading for all instrument and prospective instrument pilots.