Ball park over the fence speed calcs
 

What mental math calcs do you more experienced guys use for calculating short final approach speeds based on weight under gross when the strips a bit tight ?

1.3 x the stall speed for the configuration you are in.

The speed in the POH multiplied by the square root of (weight / MTOW).

G

Piperboy

1.3 x stall is just a number for a typical aircraft more draggy aircraft may require more more slippy aircraft less.
Its a figure to allow you enough energy to transit from a descent profile to a flare and landing and is the minimum figure quoted by the regulators

To be really accurate try flying with the ASI covered up and using an AOA gauge if any light aircraft have them :ok:
G mentioned one aircraft which had a lot of drag where the POH mentioned 1.6 plus

Pace

Pace, I actually looked at putting an aoa on the plane, there is a big push by the FAA for GA aircraft to have them fitted. I was going to wait and see if Garmin integrate there new AOA unit to show on the Garmin pilot app, like they are doing with the flightstream 210 GNS 430 setup. The current certified units seem like a helluva lot of money.

Back on subject:

So if you guys had a aircraft in this case a Maule MX7 - 180 a with a gross weight of 2400lbs and as flown on the day a weight of 1800lbs and the published VSo is 47 MPH at gross, what ball park over the fence (short final) speed would you use for a short field landing. I understand the ASI is inaccurate as all hell at thos speeds but I just was interested to see how my calcs stack up compared to what other posters would fly it at

If you reduce the weight, you reduce the lift required to support it.

As the lift increases as the square of the speed, it follows that the speed required to support a reduced weight falls with the square root of the weight. eg.

With a max Ldg weight of 2000 lbs, and a POH threshold speed of 100kt.

Reduction in weight of 10% ie. 1800 lbs would allow a reduction in speed to the square root of 0.9, ie. 0.95 of 100kt =95kt.

Reduction in weight of 20% ie. 1600 lbs would allow a reduction in speed to the square root of 0.8, ie. 0.89 of 100kt =89kt.

In this particular case, and Genghis will probably beat me up for this, one might observe that, as a rule of thumb, each 10% (200 lbs) reduction in weight will allow a 5kt reduction in threshold speed.


MJ:ok:

My ball park based on

2400 gw
1800 actual
47 mph published VS0 at gross (as per POH)

My Calcs:

600lbs under gross , knock of 1mph for every 80lbs under gross (recommended by my old instructor DHH up here in Scotland.

600/80 = 7.5 rounded down to 7 mph
VSO of 47mph minus the 7mph for weight adjustment = new VSo of 40mph
40 Mph x 1.25 ( short field approach) = 50mph short final speed.

I would have made it 52, ie. 1mph less threshold speed for every 75 lbs below 2400, as a rule of thumb. So not all that different.


MJ:ok:

How do you people hold an indicated airspeed to an accuracy of one or two MPH?

The airspeed needle is wider than that.

We didn't say we could fly it, Chuck. ;) Only that we had some way of working out what it's supposed to be.


MJ:ok:

I regularly fly into tight places, both runway, and water. If I really have to get in tight, I won't be looking at the ASI much, if at all. Similarly, for those aircraft I fly equipped with AoA, I never look at it, I'm too busy paying attention to where I'm going out the windshield.

The sight picture, relative motion of the aircraft in the vertical plane, and the feel of the plane tell me what I need to know to fly my best short approach.

If I were to refer to an ASI for a short approach, the relationship of IAS to CAS would begin to be a factor, at speeds that slow, and with the precise reference to speed being suggested. You'd best have the instrument calibration record too, as ASI's can regularly be out a few MPH/knots either way.

Having a speed worked out with the formula, based upon a CAS stall speed, and then referring to it in IAS, without consideration of instrument error could easily have you 5 MPH/knots too fast or too slow. For GA aircraft, best to just focus on flying the plane with good ground reference, and feel.

This is one of my tight landings, and I did not go eyes in at all, preferring to focus on the sight picture. This aircraft is equipped with an Alpha Systems AoA, which works really well, but aside from setting it up, I have never bothered to use it.

Amen.

Lovely strip.
I've not seen image stabilisation like that before. It's just a pity it didn't capture the actual touchdown and the Beta range turnaround was interesting. Good video, though.

I thought the question by the OP was a "Ball Park Figure" not a mental gymnastics test on finals.
As said by Pilot DAR, you shouldn't be looking in at the ASI or AoA by the time you are over the hedge, by then its sight picture, relative motion of the aircraft in the vertical plane looking out of the windshield, and the feel of the aircraft, this applies to all types of aircraft.

Good case for an AoA indicator. AoA is what the wing knows about, not speed, and the wing is what we fly. That we don't have a direct AoA readout is a major flaw in the design of most aircraft, especially aerobatic ones.

I use the AOA for high level climb in the Citation and a multitude of other things. It can tell you so much it is an amazing piece of Kit and does mean you are flying the wing and not numbers.

Light aircraft never seem to have one maybe on cost? Are there low cost units for Light GA? Pilot Dar! great video

Pace

Does buffet during aeros or high energy manoeuvring not give the best indication of AoA then you don't have to be looking in the cockpit ?

Using FLC, CLB Mode, VPATH etc these modes are using inputs from AoA, equally climbing using CAS or Mach published speeds will give you the best AoA to produce the best coefficient of lift, your airspeed indicator is measuring calibrated airspeed (CAS), which is proportional to the square root of ½ρV2 therefore indirectly is a measurement of the AoA, in other words you use them all together to get the best results.

Back on topic.

As Pilot DAR stated earlier and I fully agree by the time you are coming over the fence during the final stages of a landing the last place you should be looking is in the cockpit at the ASI or AoA, in fact when I land my light aircraft or the 100,000Lb bizjet I couldn't tell you what the speed is during the final stages of the landing because I am looking out of the window.

Going back to the OP question, "Ball park over the fence speeds" then 1.3 x CAS stall speed ( for most light aircraft, OP aircraft is a Maule ) in whatever landing configuration you have chosen will give you the safest approach speed, once approaching over the fence you should be looking for the correct sight picture and relative motion of the aircraft in the vertical plane as the ASI and AoA are really irrelevant at this stage during the landing regardless of wether its a 10,000ft runway or 250 meter grass strip.

ATC

Even using the AOA you will not be flying it to the ground :E The question is is 1.3 an accurate margin for all aircraft?

Ghengis mentioned one aircraft where the POH recommended 1.64 as it was a very draggy aircraft where 1.3 was not sufficient.

On the flip side a very slippery aircraft and 1.3 maybe too high a figure and maybe 1.2 would suffice

The AOA is a much more accurate indicator than the Airspeed indicator no one is saying at some point you do not transition from the the gauges to feel and outside references only that at what speed your VREF do you make that transition

Pace

I would say you answered your own question, use the AFM.

If no information is given then 1.3 is a good starting point.

That would depend on the type of system fitted.

ATC

1.3 is a man made figure and I think this is what the OP is getting at. The .3 being a margin in IAS above the stall in a given configuration to allow enough energy to go from a descent profile to a reduced descent profile and a flare with the throttle closed.

This margin should give enough so that you land at or near the stall.
Gs example of 1.64 means that that particular aircraft is so draggy that it requires a bigger margin.

A very slippery aircraft may mean that at 1.3 there is too great a margin.
The only way to better those fixed figures is to do a flight test on a particular aircraft, work out the correct stall figures in different configs for that particular aircraft and using 1.3 as a baseline experiment above and below that figure

A good AOA gauge is far more accurate than the ASI.

Pace

Sorry am I missing something I thought that is what I said.