2D Approach - Benefits of Using IAS vs VS Auto-flight Modes

[Sleepybhudda]

A discussion arose during a training meeting about why most similar operators use Vertical Speed as a coupled vertical mode to conduct 2D approaches. There was a counter argument that IAS was a much better mode because it controls both the speed and the angel of approach vs Vertical Speed which only controls the rate of descent.

As far as the rest of us could see both IAS and VS only control one parameter, hence their name sake. This is a turbo-prop operator and does not have a VNAV or Flight Path Angle option on the auto flight systems.

Any thoughts anyone?

[Tu.114]

Which type are we talking about? System-wise, it sounds a bit like a DH8 or similar. IAS mode on the DH8 is about the same as Levelchange, Open Clb/Des, FLCh. and would appear to me like an entirely unsuitable mode for a precise 2D approach.


In a 2D approach, the CDFA has become state of the art instead of the previous "dive and drive" method. The CDFA requires a constant descent angle down to a minimum that (in most, but not all approaches) will allow a visual continuation towards the runway. In absence of FPA or other assistances, flying a defined angle of descent is in essence a defined V/S based on a known ground speed.

Automation is there to help the crew and ease workload. So - what do those two modes offer in order to help during such an approach, or, what are the pros and cons of either mode?

IAS: the aircraft will keep a constant IAS, as the name says. The descent angle will depend on the power setting. So in order to fly a precise descent angle, an accurate power setting will be required that will result in exactly the required vertical speed. Tiny adjustments to the power setting will result in quite large fluctuations in V/S, possibly leading to pilot induced oscillations, and make a precise approach extremely hard, if not impossible. But during all this, the airspeed will be kept in a tiny range...

V/S mode: the vertical speed is a defining value in a 2D approach (again, in absence of a FPA indication), so if a required V/S is known, it is rather easy to have automation fly it. Of course, the IAS will still need to be managed using the power levers, but this is the same as on any other kind of approach and should not be particularly demanding to the crew.

I´d say V/S easily wins the contest here. Why make things more difficult than they should be...

[AerocatS2A]

Agree with Tu.114. IAS is an inappropriate mode for an approach because you end up trying to fine tune the vertical speed using the power setting and the result is imprecise and if poorly handled can lead to very high rates of descent close to the ground. Much much easier to use VS (approx groundspeed x 5) to nail the vertical profile and use the power levers to control the speed.

The argument that IAS controls both the speed and angle of approach is a poor one. In both VS and IAS one parameter is fixed and the other is controlled by the power levers. In VS the vertical speed is fixed by the VS selection and the speed is controlled by the power levers, in IAS the speed is fixed by the IAS selection and the vertical speed is controlled by the power levers. As you can see, in both cases the power levers are being used to control a parameter. It is far better to use them for speed control rather than the vertical profile when close to the ground.