Pitot Tube In-Flight Icing Design, Protection and Certification

Icing of Pitot Tubes

Pitot-static tubes (Pitots) are instruments designed to measure the static and total pressure of incoming airflow. Combining these readings, Pitots provide pilots with forward airspeed, Mach number, altitude and vertical speed that are essential for safe inflight operation.

Pitots and their static pressure ports require unobstructed contact with the surrounding air to provide accurate readings. A physical obstruction of the instruments can pose a severe danger to the flight. Examples of common obstructions are insects, airborne debris, water and ice. Inaccurate measurements from a malfunctioning Pitot system can provide pilots with incorrect readings that can lead them to react incorrectly, resulting in potentially dangerous or catastrophic situations.

For example, a blocked Pitot may cause the airspeed indicator to report an increase in airspeed as the aircraft climbs to a higher altitude, or a decrease in airspeed if the aircraft is descending, while in practice the airspeed may have remained constant. On the other hand, a blocked static port may cause the altimeter to report a constant altitude as the aircraft is ascending or descending, as well as inaccurate airspeed.

Icing of an unprotected Pitot
Icing of an unprotected Pitot after "only 20 seconds" of accretion

ANSYS FENSAP-ICE for Pitot Tube Icing Protection

Pitot tubes, being essential instruments, must be protected by electro-thermal systems when mounted on aircraft certified for flight into known icing. Their anti-icing systems are designed to standards generally provided by the airframe manufacturer based on Federal Aviation Administration (FAA) Appendix C conditions, modified by an installation factor.

The installation factor is defined as the ratio of the local liquid water content (LWC, gm/m3) at the sensor to the free stream LWC. If this factor is greater than 1 the region is labeled enriched, and if lower than 1 it is said to be diluted.

This factor is of the utmost importance, as it not only controls the power density required from the electrothermal system, but also the quantity of water to be managed by the sensor while still providing an accurate reading.

Since the Pitots must be placed outside the boundary layer of the fuselage to yield correct airspeed readings, they could be exposed to an “enriched” LWC region. This leads to higher heating requirements in order to protect the Pitots against icing. Thus, an accurate assessment of the impact of the fuselage on the local airflow is essential for the correct placement of the Pitots and the design of their anti-icing system.

Traditionally, electro-thermal heating requirements have been computed with limited-capabilities 2D icing codes. As shown in the figures, however, the flow field around the “installed” Pitot tube is highly three-dimensional.

In addition, ice blockage may come not only from ice growing on the tube, but also from ice overgrowing adjacent components. Thus, the ONLY way to correctly analyze/design/protect Pitots is to simulate the airplane as a whole, with the Pitots installed.

ANSYS FENSAP-ICE is the only system that can simultaneously analyze the aerodynamics and icing characteristics of a complete airplane and Pitots, with full 3D fidelity.

It is even more realistic than experimental simulation, as the latter cannot faithfully reproduce all geometrical or physical conditions, and should only be used for verification.

An installation factor computed by ANSYS FENSAP-ICE will be more realistic than other methods. Pitots analyzed by ANSYS FENSAP-ICE will clearly show if there is sufficient heat to melt SLD and Ice Crystals.

Airflow Stremalines Passing Over the Fuselage-mounted Pitots
Airflow streamlines passing over the fuselage-mounted Pitots, showing the complex nature of the flow and the clear impact of the fuselage: Pitot tubes must be designed in 3D, and "as installed"

Liquid water content contours
Liquid water content contours

Pitot Pressure Contours