Topic: Hail damage to aircraft

[Jimmy Deguara]

It is not a usual concern for most but hail damage to aircraft is real. I am not only referring to aircraft on the ground but also in flight! Try looking at these pictures:

http://www.srh.noaa.gov/zme/Hail_Damage.htm

photos courtesy of NOAA




Certainly does not help with the aerodynamics.

Here is a link to another website dealing with aircraft hail damage incidents:
http://www.strangemilitary.com/content/item/10136.html

Regards,

Jimmy Deguara

[Mike]

And one would think they would dodge them!

Saw a Natgeo doco the other day regarding airliners trying to land through thunderstorms - bad idea!  Not only did severe wind shear make them crash but lightning struck every single aircraft that tried to take off!

footage showed a 747 taking off and being struck by huge CG in the midsection of the aircraft.  Although even when I've flown through thunderstorms and seen flashes I've always wondered if it had struck the plane!  If you look on an aircraft's wind edges you'll see what's called 'wicks', they are short rods that protrude from the trailing edge of the wing and discharge the lightning and static electricity off the wing into the atmosphere.

Nice photos JD, would have made a racket flying through whatever they did!

Mike

[Kris Wetton]

 Flying any aircraft through or in the vicinity of thunderstorms WHERE IT CAN BE AVOIDED is contra to all and any SOP's (standard operating procedures) I have ever read. Hail is a very real risk to aircraft safety and numerous fatal accidents have occured as a direct result of hail encounters. The risk to aviation from Windshear resulting from Microburst is well documented and is now well managed, (sadly however it took fatalities, particularly the losses of a Delta airlines L1011 on approach to Dallas? and a Boeing 727 on departure from Moisant to awaken the aviation world to the dangers of shear associated with microbursts.)
With the recent advances in weather radar technology, hail encounters are now majoritively avoided, but in the 70's when the Bendix monochromatic displays were forefront technology, and our understanding and eduction relating to inclement weather encounters was average at best, it was a different story altogether and accidents DID happen.

One that springs to mind readily, in fact an accident I researched quite thoroughly, is the loss of Southern Airways flight 242, from Muscle Shoales to Atlanta, via Huntsville. The date was April the 4th 1977. In short, without the aid of sophisticated radar the aircraft entered an area of exceptionally high precipitation and hail of up to 75mm diameter! The ingestion of this hail caused the engine rpm to decrease below that required to sustain operation of the generators, the throttle levers were advanced in an attempt to obtain power, and the resulting engine surging and stalling at low rpm produced overpressures in the low pressure compressors, sufficient to result in compressor blades clashing against the stator vanes and thus causing critical damage resulting in both engines failing. Despite the efforts of the crew to glide to Dobbins AFB, the aircraft was lost when the flight crew attempted as a last resort an emegency landing on a single lane highway, resulting in the deaths of most on board.

The crew knew to expect inclement weather as the latest met they had received was:- Ceilings btwn 1000 to 2000 feet broken to overcast. with layered clouds to 15000 feet. Visibility 5 to 8km in haze and moderate precip. There were scattered Thunderstorms with tops to 35000 feet and a few of these stated as severe along the leading edge of the cold front. Moderate to severe icing existed within the thunderstorm above freezing level at 12 to 14000 feet. Gusts in excess of 50 knots and hail of 18mm or larger was a possibility with the passage of the cold front. On top of the area forecast, SIGMET's (Significant Meteorological Information, is a weather advisory that contains meteorological information concerning the safety of all aircraft in the area as desribed within the advisory) issued by met officeswere in force as well as Tornado watches issued by the National Severe Storms Forecast Centre. These predicted tornadoes E of Huntsville, with a few severe Thunderstorms reaching 58000 feet, extreme turbulence, severe windgusts to 70 knots and hail to 75mm in diameter. - NOT a good day to be flying, especially given the standard of instrumentation (typical for the period) that was on-board


Regards

Kris

[Mike]

Added to that, Kris that regardless of the year these pilots are experienced and highly trained and what about traffic controller at the time - I'm confident he would have advised them not to land there and seek alternative airports close enough even before they headed into trouble!

All airlines have SOPs as you say with regards to landing or attempting to land where thunderstorms are present of the runways or surrounds  - even the airlines that fly here in the wet do loops around thunderstorms - although last year I saw a few fly through them, but that would depend on severity of the storms anyway.

Sometimes you wonder why these blokes take the risk, knowing the risk of shear and after being told of the danger then still attempt it - beyond commonsense but i'm not going to prejudge them, they've got it hard enough already.

The air disaster you speak of, is that the accident where the aircraft flew between two severe storms in the area and whilst approaching, the storms moulded to one large severe storm?

Mike

[Kris Wetton]

Hello Mike,

No, I think you may possibly be referring to the AA accident at Little Rock Arkansas with the MD80? where the crew made some real bad judgment calls and landed in conditions well below minima with regards to RVR (Runway visibility) and X-wind

The accident neat Huntsville was also a crew induced situation, but, tempered by the fact that the Bedix monochromatics were AWFUL things in high precipitation. The crew made a turn to fly into what appeared to be a hole in the storm, according to the radar, however it was in fact an area of massive precipitation and hail called a contour hole, where the intensity of the precip prevents any radar return, It is what is referred to in aviation parlance as a 'sucker's gap'.  In this instance however, as the aircraft was already expeiencing heavy rain the returns could have been affected by attenuation. What they did in fact do, was to penetrate the worst cell and at the point of the steepest intenisty gradient.

As an aside, studies have shown that the X-band frequency used by aerial wx radar is susceptible to  attenuation by water vapour and precip! especially when covering the radome. In these circumstances, pilots could misinterpret the radar returns and is a prime example of why radar of this type should never be used as a storm penetration aid.

I have many NTSB reports from Met related incidents/accidents. Let me know if you 's where I can.

Best regards
Kris

[Mike]

A suckers gap - yes, nice analogy!  Do you have a radar image that shows a contour gap to which you are referring for interests sake?  I would have thought doppler radar would have picked up the amount of precip within that area?

Mike

[Jimmy Deguara]

Mike,

The concept of attenuation means that the intensity of particlar echos may be reduced given the shadowing of other storm echos. Doppler if they existed in the aeroplanes at that time can also pick up wind patterns. I thought during the documentary that winds were also an issue during this specific storm - not sure if it was a supercell or a squall line? I hope we have learned from our lessons from these events and not to land aircraft during severe thunderstorms over or in the vicinity of an airport.

Regards,

Jimmy Deguara