To start off we first need to know what is True Airspeed? Well, it is pretty simple it is the airspeed that your aircraft is moving through air. Now that we know that we can move onto the next portion.
Why are they different? Well the aircraft speed indicator measures pressure not speed. Your airspeed indicator will read very accurate at sea level with perfect conditions but if you add weather along with the aircraft climbing it starts reading inaccurate speeds. That is because the airspeed indicator shows a slower speed due to density increasing based on the altitude and air changes. That is the reason why you cannot directly measure true airspeed.
Now that we know why we can’t accurately measure true airspeed we need to know why does it matter? Well put simple if you use indicated airspeed to calculate fuel burn, time to destination etc you will be way off and that is why true airspeed is so important. Did you know that cruise performance charts use True Airspeed?
6-14-3 — As the altitude of aircraft increases, so will the difference between IAS and GS. Below are some very “ball-park” figures which can be used for reference. With the aircraft flying at
– 250kts IAS with no wind
– 3000ft ~ 260kts GS (+10)
– 6000ft ~ 270kts GS (+20)
– 9000ft ~ 290kts GS (+40)
– 12000ft ~ 300kts GS (+50)
There we can see a increase in speed from the set 250 knots True Airspeed as we climb through altitudes.
The Decision Altitude (DA) is a specified altitude in the Precision Approach or approach with vertical guidance at which a Missed Approach must be initiated if the required visual reference to continue the approach has not been established.
This would only be used with ILS and LNAV/VNAV approaches.
At the DA altitude a pilot must state whether they want to continue with the landing and land or go around and try again.
There are many reasons for a go around at DA.
- The pilot could be too high above glide
- Too low below glide
- To fast
- To slow
You must also only make your decision whilst still on glide, you can not dive low and then make it or fly up and make it. Your decision needs to be final and done at that point to continue the flight safely.
References: SKYbrary, BoldMethod
Minimum Descent Altitude (MDA) is a specified altitude or height in a 2D instrument approach operation or circling approach operation below which descent must not be made without the required visual reference.
A pilot can not go any lower than MDA until they see the runway. If they do not spot the runway then they will then execute a missed approach to the missed approach point (MAP).
Once you see the runway and confirmed everything you may continue your descent and land safely, if not you fly to the MAP and execute a missed approach and re-try the approach. Sometimes with very low visibility you will have to fly quite close to the runway in order to spot it and continue safely.