Accident Overview

Photo of a thunderstorm
Photo of a thunderstorm

History of Flight

TACA International Airlines Flight 110 was a regularly scheduled passenger flight between San Salvador, El Salvador, to New Orleans, Louisiana with an en route stop in Belize City, Belize. The airplane, a Boeing Model 737-300 with 38 passengers and a crew of seven on board, departed as scheduled from Belize City, and the flight was uneventful until descent into New Orleans from 35,000 feet.

During descent from 35,000 feet for landing into New Orleans Airport, the aircraft encountered severe weather. The flight crew attempted to avoid the most severe storm cells but penetrated momentarily into massive precipitation.

At approximately 16,000 feet, the airplane encountered 30 seconds of heavy hail, resulting in both engines experiencing a simultaneous complete power loss, following a thrust roll-back. The flight crew declared an emergency at 12,000 feet and radioed air traffic control (ATC) "we're in the middle of the storm....we lost an engine".

The flight crew attempted to windmill restart both engines but were unsuccessful. The flight crew then started the auxiliary power unit (APU), and was attempting to relight both engines using starter assist. In the start attempts, the engines successfully ignited, but due to the continuing presence of high concentrations of water and hail, did not accelerate to idle. The flight crew, recognizing there was no engine thrust response to throttle commands, elected to shut down both engines and prepare for an emergency landing.

The flight crew realized that a landing was not possible at the airport and started preparations to ditch. After breaking out of the cloud layer they were able to identify a small levee and were able to make a successful off-airport dead-stick landing on the levee, There were no injuries.

Photos of aerial view and ground view of levee
Photo of aerial view (left) - Photo copyright Thomas Moore - used with permission
Photo of ground view of levee (right) - NTSB docket photo

Engine Response to Heavy Precipitation

Photo of TACA flight 110 hail damage to radome
Photo of TACA flight 110 hail damage to radome
- NTSB Docket Photo

Investigation into the cause of the dual-engine flameout indicated the aircraft, while descending at low engine power and high airspeed, had encountered an area of intense rainfall followed by 30 seconds of heavy hail. The storm intensity, stated in terms of liquid water content (LWC), was estimated to be 25 to 30 grams per cubic meter, which equates to a rainfall rate of approximately 30 inches per hour. LWC is expressed in terms of the ratio of water content per volume of air, in this case grams of water per cubic meter of air.

Investigators determined that the cause of the dual-engine flameout was the ingestion of hail into the engine core. Subsequent testing revealed that, unlike rain, hail is much more effective at entering the engine core. Hail can more easily pass between the fan blades and directly enter the core, or it will bounce off the fan spinner and be deflected past the fan blades into the core.

An inlet is intended to diffuse the air flow into the engine and at high speed, slow the incoming air to align it to the fan blades. The airflow path can be influenced by the inlet and may curve relative to the airplane flight path. Unlike air, and to some extent liquid water, hail behaves in a more ballistic manner (like a bullet), and is not influenced by the diffusion effects of the inlet, maintaining a more direct path into the fan. At certain combinations of fan rotational speeds and airplane speeds, a higher concentration of hail can enter the engine, as compared to rain at the same conditions.

Weather Radar

Sample weather radar images
Sample weather radar images
Photo copyright Ajt, Inc - used with permission
(View Larger)

Diagram of TACA flight history overlaid on B737 in-flight start envelope
Diagram of TACA flight history overlaid
on B737 in-flight start envelope
(View Larger)

Modern jet transports are equipped with weather radar that displays information regarding the intensity of weather in the vicinity of the airplane. Weather radar is intended to provide the pilots with a depiction of weather in front of the airplane, and may influence flight path decisions if weather of sufficient intensity is in front of the airplane. Flight crews may decide to alter the airplane flight path based on the intensity of approaching weather. Displays typically show green for lowest intensities of weather, graduating to amber and red as intensity increases. Red areas indicate the most intense precipitation and are to be avoided. Areas where no precipitation is "seen" by the radar are displayed as black, or the same color as the display background. However, when displaying extreme precipitation levels, the radar may display the highest intensities (worse than red) as "clear" on the radar screen. This is commonly known as a "radar shadow" and is a result of attenuation of the radar signal.

Photo of turbine damage to engine - NTSB docket photo
Photo of turbine damage to engine - NTSB docket photo

Flight crews are also able to select the range to look ahead of the airplane. Range can be set as low as 5-10 miles, and perhaps as distant as 160 miles. Selection of radar range settings is important to provide the pilots the best short- and long-range picture of weather in the vicinity of the aircraft. The image on the left is a selection of sample airplane weather radar images (for example, the top right image shows a radar shadow).

Flight Crew Actions

The attempted windmill restart of both engines, due to low high pressure spool speeds (N2), in combination with continued water ingestion, resulted in excessive exhaust gas temperatures (EGT), and led to destruction of turbine components. For the TACA event, the CFM56 engine windmill in-flight restart envelope, like other turbofan engines, is based on starts in clear air conditions. However, the CFM56 windmill relight envelope is relatively small when compared to other turbofan engines of the same class.

The flight crew was successful in getting both engines to relight using the APU starter assist. However, the start sequence did not progress properly (i.e., accelerate to stable idle speeds) due to continued heavy rain ingestion. This resulted in EGTs which exceeded "red-line limits" and melted turbine components. Post-incident investigation by the engine manufacturer (CFMI) determined that it is more difficult to start an engine in precipitation than it is to continue to run during the same precipitation.

Hail Ingestion Effects

As a result of this incident, extensive testing was conducted to better understand the effects of hail ingestion on turbine engine behavior. Prior to this incident, there was widespread belief that water and hail behaved similarly when ingested into an engine. However, investigators learned that hail behaves significantly different than water when passing through an engine fan and entering the core of a turbofan engine.

Severe hail ingestion into an engine can cause adverse engine operation, such as flameout or roll-back, due to an excessive water-to-air ratio in the engine core. There are factors which contribute to the relative severity of hail ingestion effects. These include:

  • High airplane altitude increases the hail-to-air ratio
  • Low fan speed, rotations per minute (RPMs), promotes hail ingestion into the core
  • High fan speeds tend to centrifuge hail away from the core
  • High aircraft airspeeds increases the hail-to-air ratio
  • At some fan speeds and aircraft air speed combinations, ingestion into the core is greatly increased (hail misses the fan blades and goes directly into the core, commonly called the Venetian blind effect)
Diagram illustrating factors that affect hail ingestion severity
Diagram illustrating factors that affect hail ingestion severity

When the water-to-air ratio becomes sufficiently high, it overcomes the fuel-to-air ratio required for stable combustion, and subsequently results in an engine flame-out. Essentially there is too much water passing through the engine core to maintain combustion. Once the engine fails, it cannot be restarted until the water-to-air ratio decreases to a point where the fuel-to-air ratio can once again maintain combustion.

Engine Ingestion Testing

At the time the CFM56 engine was evaluated, the FAA engine hail ingestion requirements were focused on foreign object damage (FOD) to engine hardware and not engine operability effects. It was believed that conducting a severe rain ingestion test was the critical environment, and the results could also be applied to operation in continuous hail conditions. The unique behavior of hail relative to turbofan engine compressor components was not well understood at the time. It was believed that the primary threat posed by hail was foreign object damage. In addition, the method used to test some engines, involving low simulated airplane speed and high engine rotational speeds, tended to centrifuge water away from the core, thus failing to duplicate actual, critical in-flight conditions, such as those faced in the TACA incident.

The investigation estimated that the storm encountered by the TACA aircraft was the same rain intensity as that required by the engine certification standards. However, certification testing conducted on the CFM56 engine was performed with a substantially greater amount of water than was required by the regulations. Although these test demonstrated a 400% margin, they were later found to be incorrect due to the testing methods involving low airplane speed and high engine rotational speeds which tended to shield the core (i.e., centrifuging of water away from the core).

Subsequent Design Changes

Photo of a CFM56 Engine showing spinner coniptical changes
CFM56 Engine showing spinner coniptical design change
Photo copyright Andrei Nesvetaev - used with permission

Following the TACA event, CFMI conducted an extensive investigation that included multiple engine tests and detailed analysis. These tests produced a greater understanding of the behavior of hail in the compressor of the CFM56 engine, and led to several design changes.

The objective of these design changes was to increase the engine's ability to continue to run in heavy concentrations of hail and water, and included the following:

  • Spinner profile change from conical to a combination elliptical and conical (spinner shape called coniptical) to guide the hail radially outward
  • Cutback splitter that allows more ingested rain and/or hail to be centrifuged out by the fan rotor, away from the core, and into the fan bypass flow
  • Increased number of variable bleed valve (VBV) doors that allowed additional rain and/or hail to be extracted from the core flow path at low engine rotational speeds

View an animation detailing the various changes.

The FAA issued Airworthiness Directives (AD) to mandate these design changes on the CFM56 engines, and mandated Boeing 737 airplane flight manual procedure changes.

CFMI also identified and incorporated additional design changes in their follow-on engine designs that provided improvements in operation in inclement weather. These included:

  • Adaptive engine start logic for Full Authority Digital Electronic Controlled (FADEC) engines (the TACA engines were not FADEC controlled). Adaptive start logic improves the starting performance, and reduces the risk of engine damage during a start sequence.
  • Introduced improvements to the flow path contour (i.e., S - shape turn) to increase the effectiveness of VBV doors to remove material from the core flow path.

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