Centaurus
7th Aug 2015, 14:21
First, permit me to go back in time when during flying training, students in the RAAF were taught how to fly bad weather circuits at 300 feet in single engine trainers. The same principle applied to all other types, single or multi-engine. In those days (during and post war) there was no such thing as a published MDA and no height limit. The pilot was entirely responsible for terrain clearance during a bad weather circuit. The aim was to keep the runway in sight with terrain and obstacle clearance left to the pilot's judgement
Once the airport was in sight and if visibility precluded a normal circuit height, the aircraft was slowed to precautionary circuit speed and partial flap lowered to get better forward vision. That principle is unchanged over the years which is why even now at civilian flying schools, precautionary flying in assumed limited visibility requires the student to use partial flap for better visibility over the nose.
Passing abeam the landing threshold timing was started in case forward visibility was lost in rain or low cloud. Drift angle was accounted for by using the directional gyro which should have been already synchronised with the magnetic compass.
Depending on height, after 30 seconds past the end of the runway, a level turn to base leg and final was started and it was hoped that the turn if continued it would result in the airfield runway being dead ahead. If forward vision was lost due low cloud on final, runway track was maintained and a landing made if the runway came into sight. If not a go around was made on runway heading.
The timing was considered critical and keeping in close downwind was vital to keeping the runway in sight for as long as possible. But that is how it was in the old days of military flying training. As mentioned earlier, a precautionary practice circuit was usually done at 300 ft.
Fast forward to bad weather circuits (circling approach) in say a 737 as published in the 737 FCTM. Firstly, the luxury of a published circling MDA protects obstacle clearance. Secondly, it is a visual manoeuvre and if the aircraft loses forward visibility to the extent the runway cannot be seen, a specified missed approach instrument go-around procedure takes place.
With regards to the aircraft configuration for the circling approach (bad weather circuit by another name) the Boeing FCTM recommends gear down, flap 15 and flap 15 manoeuvring speed (let us assume 150 knots). The FCTM continues: " Before turning base or when initiating the turn to base leg, select landing flaps and begin decelerating to the approach speed plus wind additive." Note there is no mention of timing; maybe because the runway or runway environment should be in sight at all times.
The FCTM is a US publication and assumes the circling protected area for (say) a Category C aircraft like the 737 is an FAA TERPS figure of 1.7 nm with a max IAS of 140 knots. Also in the 737 FCTM, is the FAA Expanded circling maneuvering airspace radius that has been increased to 2.7nm if the circling MDA is 1000 ft or less. There is also a table depicting the ICAO MDA protected area for Category C aircraft as 180 knots maximum speed and 4.2nm circling area radius from the threshold.
If circling using the published FAA TERPS criteria of 140 knots and 1.7nm, it is obvious the aircraft is very close in to the runway and a continuous turn is needed to avoid over-shooting the runway centreline. That may be why the FCTM recommends landing flap before turning base or when initiating the turn to base leg in order to reduce the radius of turn.
However, in Australia it is normal to use the ICAO Cat C criteria of max IAS of 180 knots and circling area radius from the threshold of 4.2 nm. Yet some 737 operators in Australia still use landing flap from half way downwind and slow the aircraft to Vref + wind additives before turning base leg. They also time 30 or maybe 45 seconds as an arbitrary figure regardless of the published height of the MDA. Practically speaking, timing is merely whistling into the wind, since the runway should be in sight the whole time during circling.
Going back now to the opening paragraphs of this Pprune contribution. Timing of the circuit from abeam the end of the runway, originated during pre-war bad weather circuit training in case sight of the runway was lost due to low visibility. The pilot went on to instruments and continued the bad weather circuit using timing and hoping he would soon become visual again. Nowadays, with loss of sight of the runway, the regulations require the pilot to follow the published missed approach procedure. But in the old days it was standard procedure if penetrating low cloud during the bad weather circuit, to use airmanship by timing from abeam the threshold. Typically with a 500 ft bad weather circuit, timing of 15 seconds past the threshold and a level base turn would bring you out to approximately a three degree glide path on runway heading. If the circuit was flown at 1000 ft then the timing was 30 seconds past the threshold. For 1500 ft a time of 45 seconds before turning would theoretically put the aircraft on a three degree slope on final. In other words 15 seconds for each 500 ft above the runway. It was a hit and miss situation with the pilot hoping he would break out visually on final if his timing and flying had been accurate.
I understand that some operators with wide body types such as the A330 and B777 now require the whole downwind leg of a circling approach to be flown with full (landing) flap. This requires very high levels of thrust with the speed close to Vref. I don't know if this is the aircraft manufacturer’s FCTM recommendation or a company initiated procedure? I understand one domestic 737 operator circling procedure uses landing flap early downwind (needing appropriate high thrust levels). The rationale being the aircraft is in a stabilised landing configuration nice and early in the circuit. It might sound like a good idea to the uninitiated but some may consider it quite unnecessary to dirty up so early in the circuit. An engine failure in level flight with landing flap extended at Vref plus five (typically 145 knots in a 737), would require an instant reaction by the crew to prevent the inevitable rapid speed loss leading to the potential for entry into an unusual attitude at low altitude. One wonders if an engine failure in that configuration is first practiced in the simulator?
It seems to me that with Australia using ICAO protected area figures of 4.2 nm and max IAS 180 knots, there is no valid reason to be flogging downwind with landing flap out and speeds close to Vref when the normal 737 configuration of gear down and flap 15 in level flight is perfectly flyable without excessive thrust levels. Progressive flap is as required on turning final. Furthermore, it seems unbelievable that in a wide body heavy such as the 777 the configuration in a circling MDA approach is full flap at the start of the downwind leg. Is that really true with overseas airlines as well? Assuming of course circling approaches are permitted in these types.
Constructive comments appreciated
Once the airport was in sight and if visibility precluded a normal circuit height, the aircraft was slowed to precautionary circuit speed and partial flap lowered to get better forward vision. That principle is unchanged over the years which is why even now at civilian flying schools, precautionary flying in assumed limited visibility requires the student to use partial flap for better visibility over the nose.
Passing abeam the landing threshold timing was started in case forward visibility was lost in rain or low cloud. Drift angle was accounted for by using the directional gyro which should have been already synchronised with the magnetic compass.
Depending on height, after 30 seconds past the end of the runway, a level turn to base leg and final was started and it was hoped that the turn if continued it would result in the airfield runway being dead ahead. If forward vision was lost due low cloud on final, runway track was maintained and a landing made if the runway came into sight. If not a go around was made on runway heading.
The timing was considered critical and keeping in close downwind was vital to keeping the runway in sight for as long as possible. But that is how it was in the old days of military flying training. As mentioned earlier, a precautionary practice circuit was usually done at 300 ft.
Fast forward to bad weather circuits (circling approach) in say a 737 as published in the 737 FCTM. Firstly, the luxury of a published circling MDA protects obstacle clearance. Secondly, it is a visual manoeuvre and if the aircraft loses forward visibility to the extent the runway cannot be seen, a specified missed approach instrument go-around procedure takes place.
With regards to the aircraft configuration for the circling approach (bad weather circuit by another name) the Boeing FCTM recommends gear down, flap 15 and flap 15 manoeuvring speed (let us assume 150 knots). The FCTM continues: " Before turning base or when initiating the turn to base leg, select landing flaps and begin decelerating to the approach speed plus wind additive." Note there is no mention of timing; maybe because the runway or runway environment should be in sight at all times.
The FCTM is a US publication and assumes the circling protected area for (say) a Category C aircraft like the 737 is an FAA TERPS figure of 1.7 nm with a max IAS of 140 knots. Also in the 737 FCTM, is the FAA Expanded circling maneuvering airspace radius that has been increased to 2.7nm if the circling MDA is 1000 ft or less. There is also a table depicting the ICAO MDA protected area for Category C aircraft as 180 knots maximum speed and 4.2nm circling area radius from the threshold.
If circling using the published FAA TERPS criteria of 140 knots and 1.7nm, it is obvious the aircraft is very close in to the runway and a continuous turn is needed to avoid over-shooting the runway centreline. That may be why the FCTM recommends landing flap before turning base or when initiating the turn to base leg in order to reduce the radius of turn.
However, in Australia it is normal to use the ICAO Cat C criteria of max IAS of 180 knots and circling area radius from the threshold of 4.2 nm. Yet some 737 operators in Australia still use landing flap from half way downwind and slow the aircraft to Vref + wind additives before turning base leg. They also time 30 or maybe 45 seconds as an arbitrary figure regardless of the published height of the MDA. Practically speaking, timing is merely whistling into the wind, since the runway should be in sight the whole time during circling.
Going back now to the opening paragraphs of this Pprune contribution. Timing of the circuit from abeam the end of the runway, originated during pre-war bad weather circuit training in case sight of the runway was lost due to low visibility. The pilot went on to instruments and continued the bad weather circuit using timing and hoping he would soon become visual again. Nowadays, with loss of sight of the runway, the regulations require the pilot to follow the published missed approach procedure. But in the old days it was standard procedure if penetrating low cloud during the bad weather circuit, to use airmanship by timing from abeam the threshold. Typically with a 500 ft bad weather circuit, timing of 15 seconds past the threshold and a level base turn would bring you out to approximately a three degree glide path on runway heading. If the circuit was flown at 1000 ft then the timing was 30 seconds past the threshold. For 1500 ft a time of 45 seconds before turning would theoretically put the aircraft on a three degree slope on final. In other words 15 seconds for each 500 ft above the runway. It was a hit and miss situation with the pilot hoping he would break out visually on final if his timing and flying had been accurate.
I understand that some operators with wide body types such as the A330 and B777 now require the whole downwind leg of a circling approach to be flown with full (landing) flap. This requires very high levels of thrust with the speed close to Vref. I don't know if this is the aircraft manufacturer’s FCTM recommendation or a company initiated procedure? I understand one domestic 737 operator circling procedure uses landing flap early downwind (needing appropriate high thrust levels). The rationale being the aircraft is in a stabilised landing configuration nice and early in the circuit. It might sound like a good idea to the uninitiated but some may consider it quite unnecessary to dirty up so early in the circuit. An engine failure in level flight with landing flap extended at Vref plus five (typically 145 knots in a 737), would require an instant reaction by the crew to prevent the inevitable rapid speed loss leading to the potential for entry into an unusual attitude at low altitude. One wonders if an engine failure in that configuration is first practiced in the simulator?
It seems to me that with Australia using ICAO protected area figures of 4.2 nm and max IAS 180 knots, there is no valid reason to be flogging downwind with landing flap out and speeds close to Vref when the normal 737 configuration of gear down and flap 15 in level flight is perfectly flyable without excessive thrust levels. Progressive flap is as required on turning final. Furthermore, it seems unbelievable that in a wide body heavy such as the 777 the configuration in a circling MDA approach is full flap at the start of the downwind leg. Is that really true with overseas airlines as well? Assuming of course circling approaches are permitted in these types.
Constructive comments appreciated