Originally Posted by
baobab72
Hi folks
Even though these questione have been asked in the past, i was not able to locate the proper answers.
In the atr72 and bear in mind that i do not fly turboprops even though i am a huge fan of the atr, why do you have to increase the prop from 77 to 86%Np when you use the anti-ice system - if i am nt mistaken the props are left in the AUTO detent corresponding to 77%Np setting throughout the flight?
does it have to with the performance of the airplane or is it related to the load on the pneumatic system or else?
With reference to the atr crash a few years back in the chicago area due to icing accretion behind the boots, how did that result in a hinge moment reversal? My guess is that the ridge would have caused the detachment of the boundary layer across the top surface of the wing, causing the aileron to flutter and not in a full hinge reversal, but i guess the facts prove my theory wrong!
Finally why is it now recommended to hold in a clean configuration?
Kind regards
Baobab
A minimum of 86% NP is used in icing conditions because the increased rotational speed will prevent the formation of and/or improve the shedding of ice and will subsequently prevent the formation of ice aft of the deice boots in the area of the propeller slipstream. In icing incident investigations, ATR identified significant drag increases, and, in some cases, found significant decreases in lift coefficient. ATR attributed the drag increases primarily to propeller ice accretions that resulted from the propellers being operated at speeds of 77 percent, rather than the required 86 percent. According to ATR, "centrifugal effect is predominant to physically eliminate ice on the blades". "Sticking to NP 77% may lead to blade contamination resulting in drastic thrust reduction and drag increase, which could, in extreme cases, push the aircraft down to stall in level flight."
The aileron reversal happened because of the ice accumulation aft of the de-ice boots. Well aft of the boots....in front of the ailerons themselves resulting in airflow disturbance that affected the ailerons which were being held in position only until the autopilot disconnected. Testing has shown showed that ice ridge location is due primarily to the extension of flaps. When in a flaps 15 configuration at a speed close to VFE, the result is a negative outer wing local AOA.
Aileron reversal can happen without ice accumulation but only at very high angles of attack in the stall region. To ensure that this does not happen at lower angles of attack in the ATR, vortex generators have been installed in front of the ailerons to delay reversal. This initially was done on the 72 for certification and as an improvement on the 42. The "Hot and High" ATR-72-210 required an increase in maximum AOA capability. ATR subsequently added more vortex generators of a different design (co-rotative) in front of the ailerons.
The FAA has stated that "…flight testing and analysis have demonstrated that installation of vortex generators on the upper surface of the Model ATR 42 wing significantly improves the effectiveness of the ailerons, which reduces the severity of the roll upset that can occur with asymmetric ice accumulations resulting from icing conditions such as freezing
rain….". Below is a link to an old AD(AD 92-19-01) stating that the installation of VG's in front of the ailerons of the ATR42 cancels an earlier AD requiring manual flying in icing conditions, no doubt due to earlier icing related incidents.
Airworthiness Directives
Under normal ops, holding in a clean configuration while in icing is mandatory. If you are at flaps 15, the aircraft is in a more nose-down attitude allowing ice accumulation aft of the de-ice boots. However, with one engine inoperative, guess what, holding at flaps 15 in icing conditions is approved. After all, you may be below Red Bug speed. Why is this allowed? Because with one engine inop at flaps 15, you are not in as much of a nose down attitude resulting in a positive angle of attack for the outer wings so ice accumulation aft of the boots is less likely. Good thing because Red Bug can be difficult speed to achieve with one engine inop, heavy, higher elevation and in icing.
Old picture prior to VG installation....
Photos: ATR ATR-42-300 Aircraft Pictures | !!!!!!!!!!!!!!
New ATR with VG installation....There appears to be various styles with some having a large VG.
Photos: ATR ATR-42-500 Aircraft Pictures | !!!!!!!!!!!!!!
Photos: ATR ATR-42-300F Aircraft Pictures | !!!!!!!!!!!!!!
Photos: ATR ATR-42-500 Aircraft Pictures | !!!!!!!!!!!!!!