5.1.2 Hail

Hail can cause a significant loss of engine power (see Example "Leaky electronics casing", Example "Short timespan", and Example "Combination of hail and rain"). This is due to the large amount of water entering the core engine in fan engines and into the whole engine in older-model turbojets. This can affect not only the performance of the compressor, but also impair the functioning of the combustion chamber.
Hail influences the engine by changing the temperature, density, and specific heat of the air flow. In extreme cases, it can cause compressor stalls (Refs. 5.1.2-1, 5.1.2-2, and 5.1.2-3).
The high water content and corresponding temperature decrease can cause combustion to become unstable and even extinguish the flame in the combustion chamber.
There is generally no danger of mechanical damage to the fan blading from the impact of hailstones. However, resistance to hail strikes should be an important criterion for the certification of engines that are to be equipped with fiber-reinforced synthetic blades, including smaller fan engines. If a larger hailstone passes through the fan rotor in one piece, it can pose a threat to the relatively filigreed fan stator blades, which were made of light metals in early engine types, but are usually made of fiber-reinforced plastics today. In the core engine there is danger of mechanical damage to the first stator blade stage, even if it is made from titanium, since there are usually many blades with very thin cross-sections (Ref. 5.1.2-4). In fighter aircraft, the high flight speeds around Mach 1 mean that the hailstones will generally pass through the first rotor stage. These hailstones then strike the following stator assembly at corresponding high speeds. This must not result in spontaneous blade failure through fractures or deformations, which would result in a dangerous situation. Even slight damage can change the dynamic performance and dynamic strength of the blading, which can lead to dynamic fatigue fractures in the blading at a later time.
Appropriate designing of the engine intake and fan area (rotor blades, spinner) can reduce the effects of heavy hail to an acceptable level.
A further threat to engines is posed by consequential damage through parts of the engine fuselage that were damaged by heavy hail strike. For example, parts of the covering can come loose, or fragments of fiber-reinforced plastic radomes or glass shards from the cockpit could enter into the engine (see Example "Short timespan").

Example "Short timespan" (Ref. 5.1.2-5):
Excerpt: “While climbing through 20 000 feet, they encountered severe hail which lasted about 5 seconds (!), and moderate turbulence which lasted about 30 seconds. The three front windshields shattered and the radome separated from the aircraft….A landing was made at 19:40, and after inspection of the aircraft by fire department personnel, the aircraft was taxied to a gate. Postlanding examination of the aircraft by an NTSB investigator showed the radome had separated and portions of it had been ingested into the right engine. The three front windshields outer panes were shattered. The wing leading edge devices, horizontal stabilizer leading edge, vertical stabilizer leading edge, and both left and right engine inlet cowls had sustained impact damage. The left and right engine fans had sustained foreign object damage.”

Comments: The short time span of a few seconds in which the hail occurred corresponds to a traveled distance of about one kilometer at flight speed typical for that aircraft, and can be plausibly explained by a limited expansion of a storm. This also shows that special attention must be paid to the strength ratings of fiber-reinforced synthetic parts with regard to heavy hail conditions (see Figure "Vulnerable parts").

Example "Combination of hail and rain" (Ref. 5.1.2-6):
Excerpt:“…flight crew noted green and yellow returns on weather radar with some isolated red cells, left and right of intended flight path. Before entering clouds at 30 000 feet, captain selected route between two cells displayed as red on weather radar. Heavy rain, hail and turbulence were encountered. At about 16 500 feet, both engines flamed out. APU was started and AC electric power was restored while descending through about 10 600 feet. Attempts to wind-mill restart were unsuccessful. Both engines light-off by using starters, but neither would accelerate to idle; Advancing thrust levers increased EGT beyond limits. Engines were shut down to avoid catastrophic failure. Emergency landing was made on leevee (?) without further damage to the aircraft. Investigation revealed aircraft encountered level 4 TSTM (thunderstorm), but engines flamed out, though they had met FAA specs for water ingestion. Aircraft had minor hail damage; No 2 engine was damaged from overtemperature. After incident ….was issued to require min rpm of 45 % to restore use of autothrust in mod/heavy precipitation; engine modification was provided for increased capacity of water ingestion….A contributing cause of the incident was the inadequate design of the engines and the FAA water ingestion certification standards which did not reflect the waterfall rates that can be expected in moderate or higher intensity thunderstorms.”

Comments: This incident occurred in 1988 on a widely used aircraft type. A comparable case about 10 years later describes Fig. "Geometric parameters". It shows that (at least in 1988) certification regulations do not guarantee sufficient safety from problems with such environment hazards that were included as a part of the certification tests.
As remedies are considered for example an optimization of the spinner contour (Ref. 5.1.2-8) regarding the deflection of objects from the region of the core engine (Fig. "De-icing zones" and Fig. "Spinners") and the optimization of the fuel control/feeding.
Noteworthy is, that also in the other example at the right aeroengine (No. 2) start tests caused overheating failures.
Meanwhile obviously as a reaction at this incident certification and approval tests of aeroengines with a combination of hail and rain are carried out (Ref. 5.1.2-9).

Figure "Size distribution": This diagram shows the typical size distribution of hailstones in hailstorms according to Ref. 5.1.2-7. This includes the sizes of hailstones from a certification regulation from Chapter 5.1.1. It is easy to see that, for example, with maximum hailstone diameters of 2.5 cm, there will be a far larger number of slightly smaller hailstones, and single large hailstones cannot be discounted. The same is true for the specified 5 cm diameter hailstones. This gives cause to doubt the value of the given certification regulation (see Example "Combination of hail and rain").

Figure "Vulnerable parts" (Refs. 5.1.2-2, 5.1.2-4, and 5.1.2-5): There are engine areas that are at the very least indirectly threatened by the mechanical effect of the impact of large hailstones (see Example "Short timespan"). Experience has shown that the most sensitive parts are filigreed fan outlet blades and the forward stages of the low-pressure compressor (booster). Evidently, appropriate configuration of the inlet area can sufficiently minimize these risks.

Figure "Fracture behavior and strength variations": This diagram shows the differing fracture behavior and strength variations of ice samples depending on the temperature and expansion rate, according to Ref. 5.1.2-4. Decreases in temperature lead to higher ice strength and “tougher” behavior. If the expansion rate is increased, the strength increases, causing the fracture behavior to be more brittle.
Brittle behavior causes ice chunks and hailstones to burst on impact, transferring less energy into the surface they strike, meaning less stress on the affected engine part. In order to be on the safe side, i.e. to simulate the most damaging possible ice behavior, the temperature of the ice particles should be as low as possible. The fact that high strength does not necessarily cause the most damage is illustrated by the far greater damage potential of a comparable amount of water, the energy transfer of which is considerably different from that of ice.
Because the ice behaves brittle at the high impact speeds typical in the compressor, a part of the kinetic energy is lost when it bursts.

  • During the development phase and certification, the testing of engine behavior under the influence of hail should take place under the most extreme atmospheric and hail temperatures in order to maximize the FOD effect of the hailstones.
  • Experience has shown that the danger of an (at least temporary) engine failure due to extinguishing of the combustion chamber flame is highest in combined rain and hail. These conditions should be included in the performance capability verifications.
  • The design and material selection for the outlet stator blades of large fan engines must take into the risk of hail strike. This is especially true for the use of blades made from fiber-reinforced plastics and filigreed light metals. Satisfactory operation behavior requires appropriate, early, realistic verification tests of single components supported by computer simulations.
  • If there is a possibility that hail was encountered during flight, the front engine areas must be inspected for signs of damage in accordance with any available inspection and maintenance guidelines(if available). Possible damages include delamination (separation of fiber layers) in fiber-reinforced synthetic parts (e.g. spinners and outlet stator blades). This requires considerable expertise, especially with coated parts. Additionally, special attention should be given to potential damages to probes (e.g. for pressure and temperature) in the inlet area and bypass duct.


5.1.2-1 T.L. Alge, J.T.Moehring, “Modern Transport Engine Experience With Environmental Ingestion Effects”, AGARD-CP-558, proceedings of the conference “Erosion, Corrosion and Foreign Object Damage Effects in Gas Turbines”, Rotterdam, the Netherlands, 25-28 April 1994 , chapter 9, page 9-5.

5.1.2-2 H.Pant, P.M. Render, “Studies into hail ingestion of turbofan engines using a rotation fan and spinner assembly”, periodical “The Aeronautical Journal”, January 1998, Paper No. 2254, pages 45-51.

5.1.2-3 T. Tsuchiya, S.N.B. Murthy, “Water Ingestion Into Jet Engine Axial Compressors”, AIAA-82-0196, paper of the “AIAA 20th Aerospace Science Meeting”, January 11-14, 1982, Orlando, Florida, page 2-6.

5.1.2-4 J. Frischbier, “Impact Loading of Compressor Stator Vanes by Hail Ingestion”, AGARD-CP-558, proceedings of the conference “Erosion, Corrosion and Foreign Object Damage Effects in Gas Turbines”, Rotterdam, the Netherlands, 25-28 April 1994, chapter 21, pages 21-1 to 21-21.

5.1.2-5 NTSB Identification DCA98MA045, Index for May 1998.

5.1.2-6 NTSB Identification FTW88IA109, microfiche number 36079A, Index for May 1988.

5.1.2-7 M. Diem, “Die Struktur der Wolken und der Regen von der antarktischen bis zur tropischen Zone”, paper of the “2. Forschungskonferenz in Meersburg”, Germany 1967, pages 57-84.

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