Originally Posted by
megan
From the two photos it seems flaps are in the process of extending, wonder if that may have anything to do with the departure eg tailplane ice? Thoughts fdr?
Flap extension is the most problematic time for a tail plane icing event. The conditions suggest at least the last part of the flight was in VMC, what conditions existed before that BO0M has commented on. Not that long ago, right over the top of Kunming, in winter doing one of their interminable arrivals, we descended through some some interesting AS, the kind you like to see in the ski season, but not in a plane. NAI went on before entry, and we had a camera running. We picked up 3/4" of ice in 5 seconds. In 28,000 hours of driving, have never seen ice pick up that fast. Point is, there are conditions that may seem benign, and result in severe ice accretion. If that happened, it would be discernible in the FDR data as a rapid change in the drag polar, and where there may be GPS data, then the change in CL can be quickly determined. Did that happen? Hard to say (easy to drink... Johnny Walker) The data will rule it in or out.
On the SEV ICE memo, there are a lot of natural cues that a plane is carrying ice, if the crew are interested in the proceedings, I am not sure that i have ever had any alert system that has not given erroneous data at some point... Would hope that peeps driving "plains" are never basing their continued happiness and well being on any single source being good.
Originally Posted by
BO0M
"On the crash - the comment by B0OM is helpful. One question I would add: given how common you say the situation is, is there a risk of alarm fatigue with the stick shaker? How often in a pilot's career might they have the shaker activate? And what does the stall recovery procedure say to do with the flaps?"
I'd honestly say I don't know (or have seen or meet) an ATR pilot who is unaware the stick shaker has activated. It's a rather intense shaker and if crews are capable and trained correctly the muscle memory should take over and they will push. Regarding flap selection.......if clean the memo items call for Flap 15 if you have Flap 30 it stays there until the clean up post event.
Reagrding Vmca.......an ATR with an active shaker or pusher will go off well before Vmca is reached in a single engine situation. In fact to get the machine to perform (demo) the Vmca you have to deactivate the shaker and pusher and take it almost to the point of stalling. Apart from demonstration in a sim during training I have never witnessed a crew get even close to Vmca.
To the comments regarding tail plane ice. My understanding is the flight was conducted in VMC and there was no icing. If there was icing and it was bad enough to develop tail plane icing the crew would have encountered severe ice and therefore the SEV ICE memo items and checklist. That checklist limits flap to 15 for approach and landing which increases your approach speed significantly and then another factor is applied meaning you'll get nowhere near your stall speed (comment: no ATR crew in their right mind would do a circling approach after a severe icing event unless there was no other option anywhere).
If as someone suggested the ground speed was around 92 knots, this ATR was unbeliveably slow for a flap 30 config, let alone a flap 15 config. Conservative manoeuvring speeds for the ATR are 180 KIAS clean, 150 KIAS Flap 15 and Gear and 130 KIAS Flap 30. Minimum speed is 95 KCAS (period) and you'll only get close to this on an empty aircraft with no wind. The Yeti aircraft was pretty full, some basic math.............ATR empty weight 13800 kg (with crew), 68 pax with bags = 6800 kg so ZFW = 20600 kg (only 2400 kg away from MTOW if it was the 23000 kg variant). I don't know the fuel they had on board so lets just say they had a landing weight of 21000 kg. Below is the base performance data
VmHB 30 (Vref) = 107 kts
Correction = 1/3 headwind or all of the gust and that will give the Vapp. Most airlines use a stabilsation criteria of Vapp 0 - +10 some use 0 - +20
Vs Flap 30 and Gear down at 21000kg is approximately 87 Kcas
Vs Flap 15 and Gear UP at 21000kg is approximately 97 Kcas
As you may or may not see ATR gives crews approximately a 1.23 buffer (23%) Vref over Vs, its concievable if you have a 10 knot headwind that the Vapp will increase to 110 KIAS moving you further from stalling.
As a profession we seem to "double cross the bridge" on stalls and our responses to that. Seems a tad untidy, and is hardly well received by the self loaders that pay our bills. A stall is not something that we should be fearful of, even in a T tail, like the B727, Cl600 or a Lear, or an ATR, stick pusher or not. Even a Trident getting parked vertically at Staines. Yes, if you abuse the plane for long enough it will get bitey, and the recovery from that, while quite possible needs a chunk of airspace between the pointy bit and the hard lumpy bits. The F-102 is about the most notable exception to that statement, it goes from tidy to untidy really really quickly. Still has to be abused, but, golly. The Staines, BAC 1-11, B-727 accidents wee all held in the condition by the crews for longer than the plane was prepared to put up with. Don't go out and stall the planes for the heck of it, the loads imposed on the tail alone are reason enough to avoid that, but the aircraft will stall, and will still respond correctly to controls for a period of time, relaxing back pressure will break a stall, up until the point that the stall has been abused so far that the bark turns to bites. The thrust line of the ATR should provide a nose down pitching moment on application, however, as that also blows the inner wing and flaps that can become a skosh non linear. The blowing effect assists in attaching flow over the flaps at larger angles which increases lift but also increases the Cm, the downwash increase induces flow at the tail that tends to increase the nose up moment from the tail, and that can become a bit of an issue, what controls is dependent on the tail volume, arm and the geometry of the stab relative to the engines...
FDR