Pilot Techniques in Heavy Seas and Surf – 3 – Vertical Stability 

In Pilot Techniques in Heavy Seas and Surf – 1, we examined the qualities of effective hoisting: responsive, smooth, stable, and appropriately assertive. In this follow-up, I want to focus on vertical stability and share a few practical techniques that have helped me over the years. 

Because establishing a trim baseline is critical to hoist stability, I recommend first reviewing the Trim Baselines When Hoisting post if you are unfamiliar with the trim baseline concept. The tips here apply not only to heavy seas and surf hoisting, but also to any time a helicopter is moving laterally in a hover while maintaining constant altitude. 

Vertical Stability in Heavy Seas and Surf 

During hoists in heavy seas, the flight mechanic manages cable as the swimmer and survivor rise and fall with the waves. If the helicopter is also changing altitude, the situation can quickly become difficult to control. The pilot’s job is to keep the aircraft at a consistent true altitude, even as the altitude above the water fluctuates with passing swells. 

Whereas pilots can rely on the RADALT and BARALT to monitor altitude over calm water and land, respectively, both are insufficient to gauge altitude over large, moving seas. For example, over calm water, pilots can rely on the RADALT to provide an altitude above the surface with excellent precision. Over land, pilots often have fixed visual cues to indicate changes in altitude (or when the helicopter is climbing or descending). If those cues are limited and the RADALT is not helpful (e.g., during a hoist over trees or steep terrain), the BARALT can at least provide a general indication of vertical movement. Over large, moving seas, however, the RADALT cycles too rapidly to be useful, and the BARALT fluctuates almost as much because of rapid pressure changes near the surface.

Therefore, to effectively monitor altitude in heavy seas and surf, pilots need to: 

1. Set a collective force trim baseline for reference throughout the hoist. 
2. Include the Vertical Speed Indicator (VSI) in the hover scan to maintain a consistent hoist platform for the flight mechanic. Note – the MH60T VSI is instantaneous and derived primarily from the Embedded GPS/INS (EGI).

Managing Collective During Lateral Movement 

When transitioning from a zero-groundspeed hover, displacing the cyclic laterally tilts the lift vector and causes the helicopter to descend. To maintain altitude, a small, deliberate application of collective is required, and the amount of power necessary depends on the distance and duration of cyclic displacement.  

As lateral movement stops, a small decrease in power may be necessary to prevent ballooning into a climb. Once the aircraft settles from the stop, power will need to be reapplied to avoid a descent. 

Example sequence: 

• Wing down to start lateral movement → descent felt → VSI confirms → slight collective increase to hold altitude. 
• Opposing wing down to stop → climb felt → VSI confirms → slight collective decrease. 
• Motion stops → descent felt → VSI confirms→ collective returned to force trim baseline → stable hover reestablished. 

Ideally, VSI deviations should be no more than 50–100 fpm for a second or two, which equates to a manageable 1–4 ft change in true altitude. 

Three factors drive the degree of collective adjustment: 

1. Rotor tilt: Greater cyclic displacement the greater the required power correction. 
2. Groundspeed change: Small changes (1–2 kts.) need minimal power adjustment; larger changes (5+ kts.) require more noticeable collective input. 
3. Effective translational lift (ETL): If wind plus lateral groundspeed pushes the aircraft into ETL, efficiency increases, and collective must be reduced to prevent a climb. For example, in 15 kts. of wind, a slide from a hover will initially require added power, but if the helicopter accelerates through ETL, power must be taken out to maintain altitude.