Base turn Distance

[2XL]

Looking for some quick rules of thumb in relation to calculating the distance travelled (NM) during a base turn. Interested in comments on both normal circuits, and in relation to procedural instrument approaches.

[411A]

With regard to circling (TERPS, not PANOPS), the base turn generally must be within 2 nm, for catagory D aircraft.
So, from abeam (for example), plan about five miles total track distance, instrument or visual.
Works for me anyway.

[jtr]

180 kt 1.8mi

210 kt 2.1mi

Looking at x tk indicator, these work pretty well

[reynoldsno1]

There appears to be a slight confusion here between a base LEG and a base TURN.

The first is part of the visual circuit. As far as circling is concerned, the PANS OPS design criteria are larger then TERPS. Cat C 4.2NM @ 180kt IAS, Cat D 5.3NM @ 210kt IAS. The distances are predicated on a 2000ft altitude, ISA +15C and a 25kt wind factor.

A base TURN is a reversal procedure (aka a teardrop), normally part of the initial approach of an instrument procedure. The procedure starts overhead a fix or nav aid and ends when the aircraft is established inbound on final approach. The outbound track allows the aircraft to descend, and is normally limited by time (max. 3 min, though there are exceptions) or a DME distance. Max speeds from overhead the fix/aid are Cat C 240kt IAS, Cat D 250kt IAS.

[autoflight]

2XL - Try this. If your base turn is rate 1, you travel for 1 minute at your your average groundspeed. Crosswind into the circuit means the turn is 180 degrees plus twice the average drift. If you are looking at circling approach base turns, there is a lot to consider, including terrain that can kill you. There are some aircraft / configurations / airports where the GPWS will give a "too low flap" caution, a few seconds before you hit terrain on downwind.

[thermostat]

Don't have a quick rule-of-thumb but a circle is almost 3 times the diameter.
At 200 kts the radius of turn for 25 degree of bank is 1.25 nm so the diameter is 2.5 nm so the entire circle would about 7.5 nm so half of that would be 3.75 nm. It is actually 3.9 nm.
So the turn from downwind leg to final at 200 kias and 25 deg of bank is about 4 nm.
Formula for radius of turn in feet:
TAS squared divided by{(11.26) X (tangent of bank angle)}
Need a sientific calc to help.

eg : TAS 100 kts, bank angle 15 degrees
TAN of 15 = 0.267949 times11.26 =3.0171. TAS 100 X100 =10000 divided by 3.0171 = 3314 feet or .54 nm.
Half the circumference (base turn) is .54 times pie ( 3.1415) = 1.7 nm.


Hope that helps.
Thermostat.

[OzExpat]

For interest sake (read "bored" :D ), I made a few calculations, based on an altitude of 2000 feet AMSL, ISA +15, 25 degrees of bank and nil wind.

100 KIAS = 106 KTAS = 0.56 NM turn radius
125 KIAS = 132 KTAS = 0.70 NM
150 KIAS = 159 KTAS = 0.84 NM
175 KIAS = 185 KTAS = 1.07 NM
200 KIAS = 211KTAS = 1.40 NM
225 KIAS = 238 KTAS = 1.77 NM
250 KIAS = 264 KTAS = 2.18 NM

Linear interpolation should work out pretty good.

[Arctaurus]

As a matter of interest, what are the TERPS circling area dimensions for each of the categories ?

[A310driver]

RADII ARE: 1.3, 1.5, 1.7, 2.3, 4.5 nm. per TERPS

[OzExpat]

For Pans Ops, up to 2000 FT :-

1.7, 2.7, 4.2, 5.3 and 6.9 NM

[Batman]

reynoldsno1,
I agree from overhead the beacon the procedures are predicated at a max speed of 240kts, for a catC aircraft, according to Pans-Ops 8168,Vol 1, but most countries, the UK included, have a differences to Pans-Ops writen in the AIP. ie: max speed 185kts.
(see also ICAO Pans-Ops, 8168 Vol 2).

[OzExpat]

Batman ... does the UK AIP happen to state why they restrict Cat C initial approach IAS to 185 knots? What speed limit do they put on Cat D?

In PNG, Cat C and D are restricted to 200 knots IAS, with Cat A and B restricted to 170 knots - the 110/140 knot speed allowed by Pans Ops is not applied here.

[reynoldsno1]

PANS OPS is not an ICAO SARP, it is a manual offering guidelines for procedure design. However, there are SARPs covering the publishing of procedures that may require a state to file differences to ICAO standards and recommended practices.

There is nothing to stop a procedure being limited by speed and/or distance, either to avoid obstacles or minimise airspace use. I assume the latter is the main reason for the UK restriction.

[2XL]

Excellent reponses thus far so many thanks to all.

[OzExpat]

reynoldsno1 ... as a matter of interest, I am (among other things) the Senior Procedure Designer for the CAA in Papua New Guinea. I am the one who implemented Pans Ops here, complete with the speed restrictions previously posted. My interest in what the UK CAA has done with speed limits is purely professional.

Your assumption is probably quite reasonable but I'd really like to know how they justified it in their AIP. I don't have a copy of the UK AIP or I'd have investigated it more directly.

[411A]

Interesting about procedure/holding speeds. Some years ago was approaching LHR and was advised that they were in the process of changing runways and to remain at cruise altitude (FL370) and hold at LOGAN as published. LOGAN at that time was left turns at max 210 knots. Informed the controller that 210 knots for us was not possible and requested a higher speed.
His response was, '...hold as published'.
Really don't think he had ANY idea that heavy jets (or indeed any jet) need higher IAS for holding at flight levels.

[Batman]

OZEXPAT
The speed restriction of 185kts, is a terrain clearance consideration, and is applicable to all categories, unless a different speed is specified.
Look up, www.ais.org.uk. You will have to register. Section 1.9.2 refers.
Regards,
BATMAN

[reynoldsno1]

The speed restriction of 185kts, is a terrain clearance consideration

I am very surprised by this, since it applies to every procedure in the UK. I also wasn't aware that the 'terrain' in the UK was a particular problem - the topography is not exactly 'challenging' when compared to PNG, Nepal or NZ, say.

[Batman]

Yes, I agree, but thats the wording and it was a subject that was covered on my TRE check out with the CAA.
Interestingly the terrain at Glasgow makes for a 80/260 procedure turn mandatory for runway05, and for shuttle holds, the only place in the UK where this applies.

[steamchicken]

speaking of Glasgow, the Vickers Viscount was apparently designed with a very specific piece of "terrain" there in mind, at BEA's request. Take-off performance had to be sufficient to clear the giant cranes of the Renfrew (I think) shipyard, which posed a problem for some previous planes. Dealt with in Tech Log a while ago

[OzExpat]

Thanks Batman ... took a while to register and even longer to get to the relevant part of the GEN section, but finally got there. I've read it most thoroughly and can't see any reference to the speed limit being related to terrain clearance. Unless I've missed something, I suspect that you've misconstrued the situation because of the reference in the heading :

"Part III Procedure Construction and Obstacle Clearance Criteria for Instrument Approach Procedures"

This is merely a repetition of the heading used in Doc 8168. It is used there for convenience because Pans Ops is all about obstacle clearance. Whether or not significant obstacles exist in the vicinity of any particular airport or not.

I suspect that the UK CAA's differences need to be read in conjunction with their other publications. That might suggest, as reynoldsno1 said, that the reason is probably more related to tight airspace constraints than to terrain clearance.

Anyway, thanks for the reference. Seems like I could have applied a similar speed restriction here ... sure would be helpful in reducing the size of procedure protection areas!

[RHLMcG]

Should we presume that modesty no longer forbids ?

[SR71]

180 kt 1.8mi
210 kt 2.1mi
Looking at x tk indicator, these work pretty well


An interesting problem I came across last night shooting a visual into PRG onto R/W 24...

On a downwind leg at ~330kts at a DME of 8 miles & 5000', 5 miles displaced to the north of the centreline, 15kts of crosswind from the south, I wanted to know how much further I should extend downwind and subsequently what angle of bank I should aim for to ensure I rolled out on the centreline at an appropriate speed/height (i.e., gate) for the concurrent DME?

My limitations are obviously 30 degrees AOB, a radius of turn of 2.5nm and maximum gear extend 270kts whereupon the object of the exercise was to ensure that the average speed for the turn gave the appropriate rate of turn at a constant angle of bank.

In the interests of discussion, any interested parties care to share their thought processes in endeavouring to complete the above visual in minimum track miles?

:ok:

[keithl]

Batman - that 80/260 disappeared a while ago. You don't want to appear behind the times, do you!

[Roastbeef]

Hi chaps

I'm flying often a standard NDB approach. Outbound leg is 1 1/2 min on 037°, then rightturn to inbound track of 241°. this procedure is designed for Cat. B aircraft at 140kts. So this means I shall start my inbound turn after 3.5NM. Does anyone know what the range of the protected area for such a procedure is, or where I have to search for it? Thanks

[picobello]

Hey Roastbeef! What is the difference between a standard NDB app and a non standard?

:E :E :E :E :E :E :E :E

and by the way you try to fly it........

:} :} :} :}

[OzExpat]

I can't really answer your question without a few more specifics Roastbeef, but it will probably suffice to say that there is protection that includes consideration of a fairly significant tailwind (actually an omni-directional wind), plus an allowance for an error in your outbound timing, plus an allowance for the time it should take you to recognise the point at which you should turn inbound and a further allowance for the time it takes to establish the bank angle and for the plane to start turning.

There's a bit more to it than that, but that's the essence of the primary protection area. Then there's a secondary protection area around the outside of all that, to a distance of 2.5 NM. Within this area, obstacle clearance progerssively reduces to zero, so it's probably not the best idea to test the limits of the protection area too far!

I suspect that the terminology used to describe the approach, "standard", was intended to imply that it's a base turn procedure. This is soemtimes known as a tear-drop turn. I think that he (she?) was trying to differentiate between this type of approach and one that uses a procedure turn at the end of the outbound leg.

[SR71]

I'm hijacking the thread back....

FWIW, here is a table of turn radius for a combinations of bank angle (degrees) and TAS (kts):

10 15 20 25 30 35 40 45
150 1.86 1.22 0.90 0.70 0.57 0.47 0.39 0.33
180 2.67 1.76 1.30 1.01 0.82 0.67 0.56 0.47
200 3.30 2.17 1.60 1.25 1.01 0.83 0.69 0.58
250 5.16 3.39 2.50 1.95 1.58 1.30 1.08 0.91
300 7.43 4.89 3.60 2.81 2.27 1.87 1.56 1.31
340 9.54 6.28 4.62 3.61 2.91 2.40 2.01 1.68

From here it is a simple exercise to calculate the instantaneous turn rate whereupon one may verify the reasonableness of the following aide memoire (which I must confess I don't find particularly useful as I think in terms of track miles not time):

Bank Angle for Rate One turn = TAS/10+7

With the above, it is also a simple exercise to calculate track miles in the turn (~=3*turn radius), whereupon with an appropriate figure to hand of the rate of deceleration you are going to effect (clean & level + no speedbrake, clean & level + speedbrake, clean & descending + speedbrake, dirty etc etc), one may calculate the point at which to turn to hit your chosen gate.

:ok:

[Roastbeef]

thank you OzExpat,

let's call it a baseturn procedure then;)
the figure I was looking for is the 2.5NM!
I'm certainly not going to play with the margins, but some people fly this procedure some knots faster then published because of the A/C configuration! And I would like to know where the legal limitations are! Although there is no terrain around within MSA which could hit you (or is it opposite:p ) I think this is negative training.
Thank's for all the information!

[BEagle]

411A - what complete ar$e of a controller told you to hold at such an absurdly low speed?

Above 34000ft, the restriction is purely 'at or below 0.83TMN'.

It is not for nothing that such idiots have been known for years as the 'Flying Prevention Branch'!

SR71. Radius of turn = (TAS squared)/ (g x tan AoB)

Tables are for simpletons. If you need to be able to work out your turn radius, it's worth doing it properly.

[SR71]

Beagle,

The table is worked out using the formula. If you can evaluate the formula during the turn itself, credit where credit is due, but, me, being a neanderthal, I need a simple aide memoire hence the table...

The rate of turn is:

d/dt(\gamma)=V/r=g*sqrt(n^2-1)/V

where n = load factor.

n is obviously a function of the AOB.

I realised shooting the above approach I had an aide memoire for decel rate, track miles to go below FL100 but not one for turn radii at a particular speed/AOB combination...

:ok:

PS: John T - Any update on getting [tex] tag support?

[BEagle]

Why on earth try to solve a differential equation whilst flying an aeroplane?

What is it you need to know? The time it'll take to turn through a given number of degrees at a given TAS and AOB?

In which case if you want to turn through n deg, the time taken will be n x (2pi x TAS x 60)/( 360 x g x tan AoB) seconds.

At 240 TAS and 25 deg AoB, that simplifies to roughly n/2 seconds...or 0.4712 n if you prefer.

About 2/3 Rate 1

[SR71]

I asked the following a number of posts ago:

In the interests of discussion, any interested parties care to share their thought processes in endeavouring to complete the above visual in minimum track miles?

Thats what I wanted to know...

I'm quite aware of all the formulas but the point was, in the absence of an applicable aide memoire to calculate the turn radius, bearing in mind I'd rather be looking out the window on a visual than punching numbers into my calculator (as I hazard a guess you still did to evaluate the formula in your previous post? If not, you obviously have some aide memoire for calculating \tan(\phi)...), I realised I wasn't sure if my lateral displacement from the centreline was enough to ensure I didn't travel through it whilst making the turn...

Of course, in reality the differential equations do need to be solved in situ because I chose to decelerate and change the configuration in the turn. That was the whole point of the exercise in terms of reducing the track miles of the visual to the minimum by using all the appropriate aides available.

The exercise then became one of approximation...

If I started my turn at 330kts and was endeavouring to complete it at 180kts, is it geometrically possible to do so within 5 miles? (I chose to approximate such a turn by using an average of 250kts, whereupon, your/my calculations show that one only needs a lateral displacement from the centreline of 4 miles to turn through 180 degrees....even less with a crosswind in the favourable direction. Whereupon, one may relax the bank angle towards the end of the turn to roll out on the correct inbound track.)

If so, is it also possible to loose 1000' and be in the slot?

If not, what are my options?

Shall I extend further downwind and decelerate before making the turn?

So, in short, I wanted to know quite a lot of things...

:ok:

[OzExpat]

I might be missing something here SR71, but here's my simplistic idea. If you start the turn 4 NM prior to the required inbound track at a speed of about 4 miles per minute, the turn will take about a minute. Thus, if you want to lose 1,000 feet during the manoeuvre, you need a ROD of about 1,000 feet per minute.

In practice, that will work out close enough IF you can achieve an average of 1,000 FPM descent through the turn without popping pax ear drums. I assume you're flying a pressurised aeroplane so there shouldn't be a problem as long as cabin diff doesn't get excessive.

[BEagle]

Is this an attempt by the world's least favourite lo-co to work out some cowboy procedure to shave another 100kg off their sector fuel burn?

[SR71]

Beagle,

Do they go to PRG?

;)

[dicksynormous]

Personally i just fly the plate at the speed i'm supposed to and read any special brief for the proceedure. As for the need to judge height over trackmiles ,well just how do you quantify an art form as a formula.

dont over complicate your life, look at the prog page (if youve got one ) track miles. If not artform applies.

:}

Once on the gnd i tend to read the paper after i've calculated the radius of my walk around thus:

rain +1f/o= waving@drowned rat x speed of walk

So much theory and so few pilots.