Flying faster because of decreasing winds

[Chris Scott]

Like autoflight, I am increasingly dismayed at the difficulty that so many otherwise intellectually-bright forumites appear to be having to grasp what is essentially a simple concept: the importance of protecting the all-important IAS by not allowing the GS to fall too low when a negative windshear is known to be ahead.

The fascinating – but essentially esoteric – discussions about the precise definitions of inertia, momentum, and kinetic energy are unnecessary in this context, although they might well be worthy of a separate thread. On this one, they amount to little more than fog-inducing semantics. I'm wondering if, in one or two cases, they are inspired by an element of perversity of the "not invented here" variety.

The sceptics constantly emphasise the importance of IAS, and seem to recognise that it can suddenly fall dangerously, due to a loss of headwind; as described by VinRouge (post #27). Their mantra is roughly this: IAS is the only thing that matters; so we must concentrate on it exclusively, and ignore the GS. Wizofoz (post #35) seems to be implying that we, on the other hand, are advocating that GS is more important than IAS.

This is a perverse misrepresentation of my previous posts, which clearly state that the target IAS is always the higher of: the conventional approach IAS (on the one hand); and the IAS which delivers the calculated minimum GS (on the other). So we are always at an equal or greater IAS than he is.

To save readers looking back 3 days, let me remind you that in my first post (#5), I wrote: "...lift requires IAS."
I then invited sceptics to look at IAS from a fresh perspective –
"IAS = GS plus headwind-component (sea-level/ISA). Shortage of (GS) can only be corrected by applying extra thrust; for a period of time. On a bad day at the office, that time may not be available."

Two days ago, in post #16 –
"You imply that we are ignoring the all-importance of IAS. On the contrary: IAS is precisely what we are trying to conserve."
and –
"The GS-mini concept protects IAS by constantly offering the pilot an IAS target (managed speed) which results from:
the higher of VAPP and the IAS required to achieve the minimum GS."


Quote from SR71,
I've touched down in a light 737 at a GS of <90kts on a day when it was gusting 65kts. I didn't even think about GS. The only thing I seek to preserve on finals is IAS. If I fly through a shear, I need to regain IAS surely, not GS?
What am I missing?
[Unquote]

Does he want an honest answer? Seriously, though, the secret is to anticipate the shear, if you can predict it is going to happen. [If you cannot, that's another topic.] We are not discussing the situation where the wind is strong and gusty all the way down to and including the threshold; it is the sudden predicted suspension of a headwind that concerns us.

SR71 will continue concentrating manfully to regain his target IAS, which he need not have lost hold of in the first place. While doing so, he may have ample time to discuss whether his "scalar" kinetic energy, and the amount it needs to increase, is relative to an unsteady atmosphere, or to a stable platform like the earth's surface; I shall leave him to that.

Mark1234 describes the advantage of (automated) GS-mini: as the headwind falls, taking the actual IAS with it, the target IAS normally falls by the same amount (but never below the threshold IAS). That is because the inertia of the aircraft protects its GS (unlike its airspeed). And remember, TAS = GS + HWC. The minimum GS is the threshold TAS minus the headwind component of the programmed surface wind. IAS and TAS are the same at sea-level/ISA, of course, but usually not elsewhere. The system corrects automatically for the conversion of target TAS to target IAS, which is one of the awkward things to do manually, and therefore invaluable.