Low wing approach

[Genghis the Engineer]

In a well setup system, PEC (the IAS / CAS relationship) should be pretty much invulnerable to a bit of sideslip.

However, if we're talking about an elderly C172, then it may not be. There are two issues I'd be concerned about -

- one is that prior to some point in the mid 1970s, Cessna POH published operating speeds in CAS, not IAS. There was some rather odd viewpoint there that apparently pilots had nothing better to do with their time than continuous mental arithmetic to convert from CAS to IAS so that they knew what their limits and operating speeds should actually be. If you have an S model however, that bit of silliness should have been beaten out of Cessna by the time yours was built, so don't worry about it.

- the second is the possibility that the static port(s) are not positioned so as to ensure no change in error with sideslip. This is easy to check - take the aircraft up to a safe altitude (say 3000ft or so) and deliberately induce sideslip whilst maintaining the same power and pitch attitude. Monitor the ASI, altimeter and VSI. If you see a jump in alt or VSI, then you have a static nonlinearity with sideslip. If you see a jump in IAS without any change in alt or VS, then you have a pitot nonlinearity with sideslip. This last is very unlikely, and I would be very surprised if you see it.

(Do this with cruise, approach and land flap settings - the last is the one that matters but look at all of them as you may well use a flapless approach and landing with a strong crosswind after all).

If you have no pitot or static nonlinearity, then don't worry about it. If you do, then maybe it's legit to worry about it.


Except, let's go back to the reality of how you'd fly a C172 - even on an IMC IAP with a full IR coming out of cloud at the mDH of 200ft, you'll be doing the roundout and flare visually. This is an aeroplane you should be flying by visual pitch attitude and not by close reference to airspeed. So long as I had the approach speed right down to 100ft or so, and the aeroplane trimmed to that with your selected landing flaps, then get your head out of the cockpit, fly by pitch attitude, and don't obsess with airspeed - which ceases to be your friend.

Also the C172 has a good reed stall warner - which is as much your friend as any other cue the aeroplane gives you. Get yourself trimmed to Vref with flaps set by 100ft, then get your head out the cockpit and fly by attitude whilst being aware of the stall warner. If it goes off in the roundout you're too slow, if it goes off in the flare you're about right, if it doesn't go off until the wheels are on the ground, then you're too fast.

So in a nutshell, don't worry about it.

G