Low wing approach

[172510]

A question for test pilots.
From my POH (172S)
"When landing in a strong crosswind, (...). Although the crab or combination method of drift correction may be used, the wing low method gives the best control."
"Normal Approach, Flaps UP . . . . . . . . . . . . . . . . . . . . . 65 - 75 KIAS
Normal Approach, Flaps FULL . . . . . . . . . . . . . . . . . . . 60 - 70 KIAS"

When approaching using the wing low method, if I want to have the same margin above stall than when approaching using the crab method, I should fly at a higher calibrated speed. Even if I knew how much higher, I would have no idea of how much my indicated speed deviates from my calibrated speed when I fly in a slip.
In practice I would fly the approach at the higher end of the recommended approach speed range if the aircraft is heavy and I use the wing low method, and I would fly the approach at the lower end of the recommended approach speed range if the aircraft is light and I use the crab method.

Nevertheless, I'm still worried about what would happen if the calibrated speed would suddenly decrease(a gust for instance) when flying the wing low approach. I'm afraid that an asymmetric stall might be more likely to occur than when flying the crab approach.
So my question is, to what extent the crab approach is safer than the wing low approach?

[dubbleyew eight]

I'm not a test pilot but for the first half of my training I was taught to use the wing down approach. then this became less than fashionable and the crab approach was mandated. anyway I have hands on with both.

the wing down approach is just a side slip. during this you have crossed controls, say left aileron and right rudder. if you stall in this configuration you are in spin city.

in the crab approach you are just offsetting the nose to correct for drift but the controls remain in normal sense. if you stall in this config you just recover ahead with a burst of power.

both techniques work if you are practised.

the crab approach will be far safer if you lose situational awareness and stall or a gust helps you into a stall.
ymmv.

[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

[FerrypilotDK]

I was going to write an answer, but G said everything I was going to say, so I will just second it!

[twochai]

in the mid 1970s, Cessna POH published operating speeds in CAS, not IAS.

In fact, CAR 3 mandated the use of CAS markings, not IAS, for airspeed indicators. I remember questioning the rationale at the time and could never get anything sensible - I suspect it was one of those things that had "always been done that way" and nobody questioned it.

I believe it was corrected with the introduction of FAR Part 23.

[Pilot DAR]

Yes, I too have questioned limitation markings on the face of airspeed indicators, which express CAS, when there is a difference to IAS, which is where the marking appears - on an IAS scale!

Similar to twochai, no logical answer was available to explain this. It seemed that the position error was not important enough to get right on those aircraft after all....

[Genghis the Engineer]

So did other US manufacturers do the same? I can only recall ever seeing this "feature" on Cessnas.

G

[BOAC]

Golly - with all my not insignificant time on Cessna singles and twins, I thought it stood for 'Cessna Air Speed'..................

[Chris Scott]

172510 raises an interesting point, but the wing-down technique is not only applicable to light aircraft, as some contributors above know well. (Tends not to be a good idea on a/c with more than one engine mounted on each wing.)

Works well in the Dak, although in my 500 hrs I never had the chance to master it. Worked very well for me on the Dart-Herald and the BAC 1-11.

In a crosswind, the approach IAS usually has an increment of some kind added, by virtue of the strength of the wind rather than its direction. Not sure if that would be enough to compensate for the sideslip. Perhaps others more knowledgable can comment?

On the DC10-30 doing an autoland in a light-to-moderate crosswind, the autopilot initiates its own sideslip (quite early, at 138'R, IIRC) and lines the nose up on the runway QDM (i.e., the figure the pilot has set on the old-fashioned OBS). There may have been an IAS increment for autolands, but I don't have my manuals to hand. (BTW, on a long a/c it's quite difficult to decrab so as to land on the centreline, because the cockpit has to be considerably upwind prior to decrab.)

The A320 AP doesn't do that, maintaining the crab angle until just before touchdown. On a manual landing, however, despite the FBW system, it is possible (but not recommended, IIRC) to use the wing-down technique. My compromise was to induce the sideslip just before the flare, which worked well in crosswinds up to the limit. Again, an increment is likely to have been added because of the strenth of the wind.