Base turn Distance
 

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.

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.

180 kt 1.8mi

210 kt 2.1mi

Looking at x tk indicator, these work pretty well

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.

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.

base turn
 

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.

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.

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

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

For Pans Ops, up to 2000 FT :-

1.7, 2.7, 4.2, 5.3 and 6.9 NM

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).

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.

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.

Excellent reponses thus far so many thanks to all.

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.

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.

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

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.

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.

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