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QJB
1st Oct 2010, 11:58
Hi all,

I remember during my CPL training we practiced for emergency descents, in the small cessna the drill involved, closing the throttle, RPM full fine, then roll into a steep bank and let the aircraft descend in a spiral. The idea being to reduce the vertical component of lift. I was wondering would this be possible in an airliner in order to increase the descent rate in an emergency, I'm not sure what the standard procedure is but from what i gather there is no bank involved.

J

AerocatS2A
1st Oct 2010, 13:08
The benefit of a spiral descent is that you can increase the G loading which helps keep the speed down due to increased drag while maintaining a high descent rate. Airliners don't have very big G limits to start with and the passengers would find it particularly unnerving. I think the negatives would far outweigh the minimal benefits. The only real likely reason for an emergency descent is a pressurisation problem and the pilots have adequate oxygen to allow for a descent in straight flight. Having said that, it is sometimes advisable to turn off the airway so you don't conflict with traffic at lower levels but that would just be a single 90 deg turn, not a spiral descent. Also in an airliner you are typically wanting to get somewhere, a spiral descent doesn't get you any closer to your destination.

Edit: It was many years ago now, but I remember the CPL spiral descent essentially being a handling exercise, I can't recall it having any particular purpose.

Centaurus
1st Oct 2010, 14:21
I remember during my CPL training we practiced for emergency descents, in the small cessna the drill involved, closing the throttle, RPM full fine, then roll into a steep bank and let the aircraft descend in a spiral.

Sounds like a potentially dangerous exercise for no realistic purpose apart maybe if the aircraft is on fire and you need to get down to land in a hurry. A fire weakened wing will soon collapse in a tight spiral and there is not much future in that...

Pitch into fine will possibly cause a prop overspeed at Vne in a dive and in any case make little difference to the rate of descent in a Cessna single assuming a variable pitch prop of course.

The steep bank is not only quite unnecesary but again potentially dangerous especially if you happen to enter cloud and on instruments. Unusual attitude comes to mind. Finally doing a tight high speed spiral especially in IMC or at night is a recipe for severe disorienation as well as over-stressing the aircraft.

Seems to me you were taught a gimmicky procedure by a flying instructor who should have known better unless he was trying to relive old times in an airline simulator!

Checkboard
1st Oct 2010, 14:40
Spiral descent exercise for both handling, and as an emergency descent through a "hole" in the clouds, if caught flying VFR on top (which is permitted in Australia, although not in the UK), as I recall.

SURGEBLEEDVALVE
1st Oct 2010, 15:54
It can be used to cancel the effects of Negative G load when trying to initiate the rapid descent.

The bank was part of the procedure on the B707 Emergency Descent

Up to a maximum of 45 degrees bank was used to facilitate lowering the nose for a 1 G entry to a dive attitude of approximately 20 degrees.

Intruder
1st Oct 2010, 19:49
The benefit of a spiral descent is that you can increase the G loading which helps keep the speed down due to increased drag while maintaining a high descent rate.
Actually, if you increase the G loading, you'll decrease the rate of descent. The object is to REDUCE the vertical component of lift, and a 45 deg bank will reduce it to .707G. That will give a greater descent rate at the same IAS and 1G apparent.

A turning descent can be advantageous near an airport where initial course for return is not important, and also to keep the airplane inside a smaller lateral area.

FCeng84
1st Oct 2010, 23:28
The objective during an emergency descent is to discipate energy as fast as you can. This means drag - lots of it. Drag is increased by increasing speed and by increasing load factor. The steeper the bank, when maintaining a relatively constant vertical flight path angle, the higher the g and thus the greater the drag.

The FARs include a requirement for maximum cabin altitude:
- never above 40K feet
- above 25K feet for no more than 120 seconds

The crux is to make sure that you can get down to 40K feet before the cabin altitude increases to that level and to get down to 25K feet in less than two minutes.

While a turning descent would improve performance (i.e., reduce time to descend) commercial transports are certified using a straight ahead method. The first part is throttles to idle and a moderately agressive pitch down (0.5g) to quickly accelerate to Mmo/Vmo. You have to be careful approaching the altitude of the Vm0/Mmo corner lest you overshoot Vmo. Descent gradient along Mmo is much steeper than along Vmo.

Exaviator
2nd Oct 2010, 00:16
Commencing an emergency descent by rolling into a 30 degree banked turn and pitching down at the same time is a common method used by many airlines. It not only prevents any negative G - keeps the passengers in their seats and turns the aircraft off the airway, thereby reducing the chance of a collision.

With no structual damage descent is made at MMO, with throttles closed and full speed brake. If damage is suspected a low speed option is available reducing to gear extension speed and lowering the undercarriage, also with throttles closed and full speed brake.

In either configuration descent rates remain about the same.

Prior to commencing the above the flight deck crew will have donned their full face oxygen masks and established crew communication and as the cabin altitude climbs above 10,000ft pax oxygen masks will have deployed in the cabin.

It is a straight forward exercise which is carried out regularly in the simulator. :ok: