Q2. Short Field Speeds - Why is the approach figure always the same?

[SIMUL8D]

A quick one - I hope?

I used to fly Caribou aircraft (aka CCO8, or DHC-4) with the RAAF. We had approach speeds that would vary based upon current, real-time aircraft weight.

Now I'm teaching Short Field Technique on a C172SP. The Lift-off and Obstacle clearance speeds vary with weight in the POH, but the approach speed is always 61kts. Why?

I'm guessing that it may have something to do with certification? Perhaps an allowance for mishandling?

I hope I haven't just embarrassed myself.... but they do say there are no stupid questions in aviation - don't they? Just bad spelling probably

I look forward to your responses.

Cheers

[A and C]

I think it is because the Vref changes so little with change in weight in a light aircraft that the certification authoritys don't think it is worth the trouble of producing the data.

[SIMUL8D]

Ok. That's possible.

The POH has calculated weights and obstacle clearance speeds for the Short Field Take-off as follows;
2550 Lift-Off 51KIAS Climb out 56KIAS
2400 48 54
2200 44 50

Short Field Landing is only calculated at 2550, and only states 61 knots to the Flare.

Stall speed in the C172SP is 40KIAS with Flap 30. The Stall speed is calculated at MAUW and most forward CoG.

61 knots to the flare feels 'hot' and you have to float to achieve an acceptable landing attitude. Don't get me wrong. I'm not suggesting doing anything that's not in the POH - but I would like to know from whence the figure is derived?

[Whopity]

I would be more inclined to use 55 kts for a short field in a 172SP than 61kts. a 33% margin on 40 gives 53kts. I have brought it back to 45 kts for a spot landing.

[Tinstaafl]

When you say 61kts to the flare, do you mean a point around 10' or 20' or so? If so then that could be an issue. Cessna, Piper & Beech typically specify a glide at the nominated speed through the 50' screen height. After that no expectation to maintain the speed to the ground.

[DFC]

There are a number of issues here.

First and foremost, one must at all times operate the aircraft in the maner specified by the manufacturer. If the POH / AFM specifies an approach speed then that is the speed to use. In particular one should not use a lesser speed because one is not aware of why that speed was specified.

Stall speed is only one element in deciding the approach speed. Elevator effectivenees is another and there is little point in approaching at 1.3 times the stall speed if it breaks the nose wheel because the elevator is not effective enough in the worst case to raise the nose for a mainwheel touchdown.

It is the Manufacturer who decides what approach reference speed to use (Vref). The regulations require that this speed is not less than 1.3 Vso.

The landing performance will have been tested and calculated using the chosen speed as a minimum, as well as the specified configuration and technique.

The manufacturer will decide the target market for their product. If setting a single speed for landing and the resultant landing performance satisfies that ,market then it is a waste of time, effort and money to test, check and certify other speeds.

For the landing distance, this is determined from a steady approach at a gradient of not greather than 5.2% (3 degrees) at a speed not less than the Vref to 50ft at which point the throttle is closed.

This is often referred to as "close the throttle when the last obstacle has been cleared".

So please remember that the manufacturer's procedures are usually based upon a situation as described - no wind level pabed dry runway etc etc and that the approach speed is only maintained down to the 50ft point where the throttle is closed and a power off landing on the mainwheels is made followed by maximum braking.

If you want to see the profile, draw a 3 degree approach to 50ft. Then measure the Landing Distance Published. From the end of the Landing distance measure back the published Ground roll figure and then join the start of the ground roll to the 50ft point. You will often find that this is a lot steeper than 3 degrees!!

So it is not the approach that is steeper than 3 degrees, it is the initial part of the landing. This is why many pilots can't figure out why they are not coming close to the publsihed figures. Flying the 3 degree slope to (close to) the ground eats up 1000ft from the 50ft point and is not what the manufacturer has described.

In sumary, the answer to the original question is that the manufacturer probably can't be bothered spending the money certifying several different speeds when there is no market for them.

------------

If one flies a standard club approach of 3 degrees all the way down to a low flare then it is simple math that 3 degrees eats up in round figures 1000ft from the 50ft point.

Now looking at a (not a 172) landing distance table for a light aircraft;

Ground roll = 445. Total to clear 50ft obstacle = 1075.

If one flies the stable power on 3 degree slope down to the low flare, one needs to at a minimum use 1000 + the ground roll figure as the landing distance i.e. 1445. This is a 34% increase on the published figure. That alone eats up a big chunk of the 43% safety factor that the CAA recomend we use. In fact one is left with a true safety margin of 6%.

This I hope emphasises the fact that unless one uses the exact manufacturer procedure then it is unlikely that one will acheive the scheduled performance.

[Trim Stab]

A quick one - I hope?

I used to fly Caribou aircraft (aka CCO8, or DHC-4) with the RAAF. We had approach speeds that would vary based upon current, real-time aircraft weight.

Now I'm teaching Short Field Technique on a C172SP. The Lift-off and Obstacle clearance speeds vary with weight in the POH, but the approach speed is always 61kts. Why?

On the Caribou (as in any multi-engine aircraft) you fly the approach at or above Vapp, which is the minimum airspeed needed to successfully execute an N-1 missed approach at DH. It is a safety speed which allows you (in the worst case scenario) to initiate a go-around at DH, simultaneously lose one engine, and still have sufficient airspeed to remain above Vmca as you pitch up, get the gear and flaps up, and look for the blue line. Vapp varies according to the weight of the aircraft because the performance of multi-engine aircraft at N-1 is generally fairly marginal so a small change in weight can significantly change the N-1 climb performance.

If you do not go around at DH, and continue the approach, you then decelerate to Vref (1.3Vso) at the threshold.

In a single-engine aircraft none of this applies since you have plenty of reserve power for a go-around, and the climb performance is therefore proportionally less affected by weight. Hence in a single-engine you can fly the whole approach at Vref (or 1.3vso) if you wish.

[DFC]

Hence in a single-engine you can fly the whole approach at Vref (or 1.3vso) if you wish.

That could be very unsafe especially if the manufacturer has set the minimum approach speed higher than 1.3 Vso.

Please remember that the approach speed selected by the manufacturer for certification can not be less than 1.3 Vso. It can be more.

The performance A aircraft (like the citation) does not have a single engine comittal height. Therefore it is entirely possible (and safe) to fly a single engine approach and execute a missed approach at 1ft. Are you saying that you would fly Vapp all the way down if you were OEI?

If you want to talk multi engine you need to pay more attention to Vmca and Vmcl which may be more critical than Vso

[Pull what]

Stall speed is only one element in deciding the approach speed. Elevator effectivenees is another and there is little point in approaching at 1.3 times the stall speed if it breaks the nose wheel because the elevator is not effective enough in the worst case to raise the nose for a mainwheel touchdown.

Also aileron effectiveness is taken into consideration. DFC is correct, follow the manufactures advice, at least you can quote it in court, if you survive!

[Big Pistons Forever]

This thread like most others on this forum, has migrated to a esoteric theoretical discussion entirely removed from the real world. The bottom line is quite simple. Since the average light aircraft needs about 50% more runway to takeoff than to land achieving the absolute minimum landing distance is rather pointless. In any case for the average PPL the problem is not adjusting the approach speed for weight of the aircraft..... it is setting the attitude and power setting to give a stabilized smooth flight path at a steady airspeed to the desired touchdown point in the first place

Finally despite what the POH says anybody who has any time flying a C172 knows that at very light weights (one person and half tanks) 55 kts works better for an approach speed.

[Centaurus]

I used to fly Caribou aircraft (aka CCO8, or DHC-4) with the RAAF. We had approach speeds that would vary based upon current, real-time aircraft weight.

Now I'm teaching Short Field Technique on a C172SP. The Lift-off and Obstacle clearance speeds vary with weight in the POH, but the approach speed is always 61kts. Why?

You have to go back to the war years. The manufacturer through qualified test pilots published approach speeds in Pilot's Notes. For example, Pilots Notes Mustang page 35 stated with flaps fully down the engine assisted approach speed was 100 knots and in a glide approach 115 knots. The latter allowed for the high rate of descent with idle or no power and allowed extra energy for the flare. For a flaps up landing the approach speeds were 110 knots power assisted and 120 knots no power.

Pilot's and Flight engineer Notes for the Lincoln Mk 1 and 2 recommended final approach speeds were at max landing weight engine assisted 95 knots and glide 105 knots while at light load (56,000lbs) it was 90 knots and 100 knots.

In those days Pilot's Notes did not publish landing distances and it was common practice to land on an all-over grass airfield that could be awfully short. The pilot would then on short final, steadily reduce the speed by 10-15 knots from the normal recommended approach to just above the stall. In fact if all power was pulled off too early the aircraft would stall. A carrier landing in the Sea Fury was a good example. From Pilot's Notes Sea Fury: The recommended final approach speeds at max landing weight of 14,000lbs with flaps down, engine on is 110 knots and with flaps up 125 knots. However for a deck landing the recommended speed is 90-92 knots and it is necessary to pull the control column well back to effect a three-point landing.

In other words there are normal recommended approach speeds and short field landing approach speeds. With the Sea Fury it would be dangerous to chop the power at 92 knots until the flare was initiated as the propeller slipstream would reduce and loss of lift occurs.

From this we can see that a short field landing or precautionary landing was conducted in most aircraft at a speed just above the stall. This resulted in no float, a firm touch down in three-points in a conventional (tail-wheel type) and minimum landing run. because that was the point of the exercise - ie landing in the shortest distance so brakes could be applied as soon as possible. One typical example was the Lysander high wing fixed gear aircraft used for dropping off agents into enemy territory on unknown length grass fields. Approaching over obstacles into short fields required the same slow speed technique.

This wartime technique carried over to civilian flying schools and for many years until American light trainers such as the Cessna 150 and Warriors arrived, short field precautionary landings were taught to ab-initio students as part of their PPL training. The American built trainers were subject to FAA certification and the manufacturer's Pilot Information Manuals were required to show landing length performance figures based upon recommended approach speeds to attain those minimum landing lengths. Cessna called their tables Short Field Landing lengths even though the speeds recommended were "normal" speeds and not 15 knots knocked off to make sure.

Despite the American influence with its accent on published landing lengths and commensurate safe approach speeds, many flying schools in Australia (for example) published in their home-grown operations manuals, short landing speeds well below "normal" landing speeds. It was intended for pilots to fall back on these slower speeds if there was a requirement to make a power-on forced landing into a farmer's field following a precautionary search. One such aero club manual published in the Sixties gave the Cessna 172 final approach speed as 60 knots with full flap. However for a precautionary landing the manual stated that at 300 feet on final the speed should be reduced to 50 knots and sufficient power to control rate of descent right down to just above the ground, where the throttle is closed and final round-out made. Exactly the same procedure was used for aircraft such as the Tiger moth and Chipmunk.

For many years short field landings were taught at RAAF CFS during QFI courses and at the various Service flying schools. In turn, it was customary to leave it to the pilot’s discretion as to what final short field landing speed was considered appropriate on other advanced types. In general it was considered reasonable to reduce the normal (1.3Vs) final approach speed by say 10 knots in order to achieve minimum ground roll – which after all was the whole purpose of short field landings.

But things have changed a lot since those days and now the manufacturer’s Pilot’s Information Manuals publish recommended final approach speeds that meet certification requirements and the wartime days of the real short field landing where 10-15 knots was knocked off on final and the aircraft was almost literally hanging on the prop until the flare, are long since gone. Interestingly, one young inexperienced instructor was asked what speed he taught for the “short field” landing and he said he used the stall warning horn as a guide. In other words he counted on the intermittent sound of the stall warning to keep the desired speed. In fact he was teaching what all flying instructors taught in the old days of knocking off 10-15 knots on short final. Except there were no stall warning devices fitted to those early types. The Tiger Moth had automatic slats or were they “slots” that fluttered in and out with a loud clacking noise when near the stall. A reliable stall warning device it was too.

The problem with current flying school training is that many students are still taught that the normal approach speed published in the POH is termed “short field”. They are told it is also the speed used for a precautionary landing speed. Furthermore the students are briefed that the speed is so slow and critical they can expect no float and the aircraft will fall out of the sky if power is reduced on final. Of course that is quite wrong. It stems from the fact that many instructors teach a faster speed on short final to allow for student handling errors and when the flight manual certification speed is used this is then termed a short field landing. It is all a matter of history.

[Chuck Ellsworth]

After a lifetime of flying airplanes one thing I have learned is those who quote rules, regulations and use hard numbers such as speeds accurate to one knot as cast in concrete guidlines to safely and skillfully fly are almost always the most inept at actually flying an aircraft.

[Pull what]

After a lifetime of flying airplanes one thing I have learned is those who quote rules, regulations and use hard numbers such as speeds accurate to one knot as cast in concrete guidlines to safely and skillfully fly are almost always the most inept at actually flying an aircraft.

After a similar lifetime I have learnt that most people on forums are so full of **** they need an enviromental licence

[Kiltie]

Brilliant! Well said! Pprune is so full of smug self-congratulating 100,000,000-posts n*bbers these days it's about time someone spoke up.

[Tee Emm]

After a similar lifetime I have learnt that most people on forums are so full of **** they need an enviromental licence

Well done, that man. Your erudite discourse had added to the tenor of this discussion...

[Runaway Gun]

Still, I think Centaurus' post was a great read !!

[Chuck Ellsworth]

Brilliant! Well said! PPRuNe is so full of smug self-congratulating 100,000,000-posts n*bbers these days it's about time someone spoke up.

Yes, the Achilles heel of these forums is the anonymity factor.

Most of the experts who berate the commoners among us do so from the comfort and protection of the cave they are hiding in.

Some of us at least have the self worth to not hide behind anonymity, and do call them on their attitude and at times even tell them to go .... them-self's in code.

[Tee Emm]

Most of the experts who berate the commoners among us do so from the comfort and protection of the cave they are hiding in.

Often true. On the other hand some contributors reluctantly elect to user pseudo names because of perceived job security issues. Keep in mind, Pprune is widely read by those in authority and even regulators. There is no shortage of principled but unemployed pilots who at their personal cost naively thought their views on flight safety matters would be listened to - but regret now speaking out. With that in mind is it any wonder that pseudonyms are used? A necessary evil, perhaps.

[Chuck Ellsworth]

I don't know if it is a necessary evil or not.

Maybe I am a victim of my upbringing from a past era when we were taught that personal integrity was a virtue.

Regardless the world is the way it is and seems to be getting less attractive.

As an aside I have been posting on these forums for some years and have been the target of a few viscous comments from people who choose to be anonymous, I take the comments from where they are coming and let those who read these forums make their own assessment of what is said.

The secret to not becoming a target for actions from a regulator is don't make statements that give them ammunition to use against you.

As to what other posters may think of you just think of it as doing them a favor.......when they see you at the airport they can proudly tell their friends " There's that axxhole Ellsworth who posts on Pprune ".

[Tmbstory]

Personal Integrity

Chuck Ellsworth:

I agree with your comment about Personal Integrity, I wish that there was more of it today.

Some people say that you should fly at a different standard when you carry passengers. I have always disagreed with this concept and used my own standard all the time. My rationale was that if I had the good smooth flight then the people behind me also had the best flight.

Tmb